How to stop exception from searching Unavailable/no cities?

I have a Weather app that searches for any city typed on the EditText and it works very well. But the problem is that the app crashes and reports "(my app) has stopped" on my phone whenever I search for an unavailable city or I leave the EditText empty and click the search button(this only happens when I search an unavailable city/no city is searched), It can search any valid city correctly.

Here's what my Logcat displays(only when an unavailable city is searched):

java.lang.NullPointerException: Attempt to invoke virtual method 'com.viz.lightweatherforecast.Retrofit.Example$Main com.viz.lightweatherforecast.Retrofit.Example.getMain()' on a null object reference
        at com.viz.lightweatherforecast.FirstFragment$1.onResponse(FirstFragment.java:104)
        at retrofit2.DefaultCallAdapterFactory$ExecutorCallbackCall$1.lambda$onResponse$0$DefaultCallAdapterFactory$ExecutorCallbackCall$1(DefaultCallAdapterFactory.java:89)
        at retrofit2.-$$Lambda$DefaultCallAdapterFactory$ExecutorCallbackCall$1$3wC8FyV4pyjrzrYL5U0mlYiviZw.run(Unknown Source:6)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6819)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:497)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:912) 

and

java.lang.NullPointerException: Attempt to invoke virtual method 'com.viz.lightweatherforecast.first.PrettyTime com.viz.lightweatherforecast.Retrofit.Example.getDt()' on a null object reference
        at com.viz.lightweatherforecast.Activity.HomeActivity$1$2.onResponse(HomeActivity.java:100)
        at retrofit2.DefaultCallAdapterFactory$ExecutorCallbackCall$1.lambda$onResponse$0$DefaultCallAdapterFactory$ExecutorCallbackCall$1(DefaultCallAdapterFactory.java:89)
        at retrofit2.-$$Lambda$DefaultCallAdapterFactory$ExecutorCallbackCall$1$3wC8FyV4pyjrzrYL5U0mlYiviZw.run(Unknown Source:6)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6819)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:497)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:912)

when no city is searched(EditText is empty).

So I'm trying to stop the exception responsible for that and as well write a toast/message to tell the user "no city found" when an unavailable/no city is searched. Please how do I do that? So far, I've tried adding:

Log.d(TAG, "No City found"); 

on my

@Override
                    public void onFailure(@NotNull Call<Example> call, @NotNull Throwable t) {
                        t.printStackTrace();
                    }

the method in HomeActivity, but it still crashes.

Here's my specific code:

HomeActivity.java:

public class HomeActivity extends AppCompatActivity {
    // User current time
    TextView time_field;
    ImageView Search;
    EditText textfield;
    ConstraintLayout constraintLayout;
    // For scheduling background image change
    public static int count=0;
    int[] drawable =new int[]{R.drawable.dubai,R.drawable.central_bank_of_nigeria,R.drawable.eiffel_tower,R.drawable.hong_kong,R.drawable.statue_of_liberty};
    Timer _t;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_home);

        time_field = findViewById(R.id.textView9);
        Search = findViewById(R.id.imageView4);
        textfield = findViewById(R.id.textfield);

        BottomNavigationView bottomNavigationView = findViewById(R.id.bottomNavigationView);
        final NavHostFragment navHostFragment = (NavHostFragment) getSupportFragmentManager().findFragmentById(R.id.fragment);
        assert navHostFragment != null;
        final NavController navController = navHostFragment.getNavController();
        NavigationUI.setupWithNavController(bottomNavigationView, navController);

        Search.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                getWeatherData(textfield.getText().toString().trim());
                FirstFragment firstFragment = (FirstFragment) navHostFragment.getChildFragmentManager().getFragments().get(0);
                firstFragment.getWeatherData(textfield.getText().toString().trim());

                            constraintLayout = findViewById(R.id.layout);
                            constraintLayout.setBackgroundResource(R.drawable.dubai);
                            _t = new Timer();
                            _t.scheduleAtFixedRate(new TimerTask() {
                                @Override
                                public void run() {
                                    // run on ui thread
                                    runOnUiThread(() -> {
                                        if (count < drawable.length) {

                                            constraintLayout.setBackgroundResource(drawable[count]);
                                            count = (count + 1) % drawable.length;
                                        }
                                    });
                                }
                            }, 5000, 5000);
                        }

            private void getWeatherData(String name) {

                ApiInterface apiInterface = ApiClient.getClient().create(ApiInterface.class);

                Call<Example> call = apiInterface.getWeatherData(name);

                call.enqueue(new Callback<Example>() {
                    @Override
                    public void onResponse(@NotNull Call<Example> call, @NotNull Response<Example> response) {

                        assert response.body() != null;
                        time_field.setText("Last Updated:" + " " + response.body().getDt());

                    }

                    @Override
                    public void onFailure(@NotNull Call<Example> call, @NotNull Throwable t) {
                        t.printStackTrace();
                    }

                });
            }

        });
    }
}

FirstFragment.java:

public class FirstFragment extends Fragment {
    // User current time, current temperature, current condition, sunrise, sunset, temperature, pressure, humidity, wind_speed, visibility, clouds
    TextView current_temp, current_output, rise_time, set_time, temp_out, Press_out, Humid_out, Ws_out, Visi_out, Cloud_out;
    // TODO: Rename parameter arguments, choose names that match
// the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
    private static final String ARG_PARAM1 = "param1";
    private static final String ARG_PARAM2 = "param2";

    // TODO: Rename and change types of parameters
    private String mParam1;
    private String mParam2;

    public FirstFragment() {
        // Required empty public constructor
    }

    /**
     * Use this factory method to create a new instance of
     * this fragment using the provided parameters.
     *
     * @param param1 Parameter 1.
     * @param param2 Parameter 2.
     * @return A new instance of fragment SecondFragment.
     */
// TODO: Rename and change types and number of parameters
    public static FirstFragment newInstance(String param1, String param2) {
        FirstFragment fragment = new FirstFragment();
        Bundle args = new Bundle();
        args.putString(ARG_PARAM1, param1);
        args.putString(ARG_PARAM2, param2);
        fragment.setArguments(args);
        return fragment;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (getArguments() != null) {
            mParam1 = getArguments().getString(ARG_PARAM1);
            mParam2 = getArguments().getString(ARG_PARAM2);

        }
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View rootView = inflater.inflate(R.layout.fragment_first, container, false);
        // For displaying weather data
        current_temp = rootView.findViewById(R.id.textView10);
        current_output = rootView.findViewById(R.id.textView11);
        rise_time = rootView.findViewById(R.id.textView25);
        set_time = rootView.findViewById(R.id.textView26);
        temp_out = rootView.findViewById(R.id.textView28);
        Press_out = rootView.findViewById(R.id.textView29);
        Humid_out = rootView.findViewById(R.id.textView30);
        Ws_out = rootView.findViewById(R.id.textView33);
        Visi_out = rootView.findViewById(R.id.textView34);
        Cloud_out = rootView.findViewById(R.id.textView35);

        return rootView;
    }

    public void getWeatherData(String name) {

        ApiInterface apiInterface = ApiClient.getClient().create(ApiInterface.class);

        Call<Example> call = apiInterface.getWeatherData(name);

        call.enqueue(new Callback<Example>() {
            @Override
            public void onResponse(@NotNull Call<Example> call, @NotNull Response<Example> response) {

                assert response.body() !=null;
                current_temp.setText(response.body().getMain().getTemp() + " ℃");
                current_output.setText(response.body().getWeather().get(0).getDescription());
                rise_time.setText(response.body().getSys().getSunrise() + " ");
                set_time.setText(response.body().getSys().getSunset() + " ");
                temp_out.setText(response.body().getMain().getTemp() + " ℃");
                Press_out.setText(response.body().getMain().getPressure() + " hpa");
                Humid_out.setText(response.body().getMain().getHumidity() + " %");
                Ws_out.setText(response.body().getWind().getSpeed() + " Km/h");
                Visi_out.setText(response.body().getVisibility() + " m");
                Cloud_out.setText(response.body().getClouds().getAll()+ " %");
            }

            @Override
            public void onFailure(@NotNull Call<Example> call, @NotNull Throwable t) {
                t.printStackTrace();
            }
        });
    }
}

EDIT

Example.java:

public class Example {

    @SerializedName("coord")
    private Coord coord;
    @SerializedName("weather")
    private List<Weather> weather = null;
    @SerializedName("base")
    private String base;
    @SerializedName("main")
    private Main main;
    @SerializedName("visibility")
    private Integer visibility;
    @SerializedName("wind")
    private Wind wind;
    @SerializedName("clouds")
    private Clouds clouds;
    @SerializedName("dt")
    @Expose
    private PrettyTime dt;
    @SerializedName("sys")
    private Sys sys;
    @SerializedName("timezone")
    private Integer timezone;
    @SerializedName("id")
    private Integer id;
    @SerializedName("name")
    private String name;
    @SerializedName("cod")
    private Integer cod;

    public Coord getCoord() {
        return coord;
    }

    public void setCoord(Coord coord) {
        this.coord = coord;
    }

    public List<Weather> getWeather() {
        return weather;
    }

    public void setWeather(List<Weather> weather) {
        this.weather = weather;
    }

    public String getBase() {
        return base;
    }

    public void setBase(String base) {
        this.base = base;
    }

    public Main getMain() {
        return main;
    }

    public void setMain(Main main) {
        this.main = main;
    }

    public Integer getVisibility() {
        return visibility;
    }

    public void setVisibility(Integer visibility) {
        this.visibility = visibility;
    }

    public Wind getWind() {
        return wind;
    }

    public void setWind(Wind wind) {
        this.wind = wind;
    }

    public Clouds getClouds() {
        return clouds;
    }

    public void setClouds(Clouds clouds) {
        this.clouds = clouds;
    }

    public PrettyTime getDt() {
        return dt;
    }

    public void setDt(PrettyTime dt) {
        this.dt = dt;
    }

    public Sys getSys() {
        return sys;
    }

    public void setSys(Sys sys) {
        this.sys = sys;
    }

    public Integer getTimezone() {
        return timezone;
    }

    public void setTimezone(Integer timezone) {
        this.timezone = timezone;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getCod() {
        return cod;
    }

    public void setCod(Integer cod) {
        this.cod = cod;
    }

    public class Clouds {

        @SerializedName("all")
        private Integer all;

        public Integer getAll() {
            return all;
        }

        public void setAll(Integer all) {
            this.all = all;
        }

    }

    public class Coord {

        @SerializedName("lon")
        private Double lon;
        @SerializedName("lat")
        private Double lat;

        public Double getLon() {
            return lon;
        }

        public void setLon(Double lon) {
            this.lon = lon;
        }

        public Double getLat() {
            return lat;
        }

        public void setLat(Double lat) {
            this.lat = lat;
        }

    }

    public class Main {

        @SerializedName("temp")
        private Double temp;
        @SerializedName("feels_like")
        private Double feelsLike;
        @SerializedName("temp_min")
        private Double tempMin;
        @SerializedName("temp_max")
        private Double tempMax;
        @SerializedName("pressure")
        private Integer pressure;
        @SerializedName("humidity")
        private Integer humidity;

        public Double getTemp() {
            return temp;
        }

        public void setTemp(Double temp) {
            this.temp = temp;
        }

        public Double getFeelsLike() {
            return feelsLike;
        }

        public void setFeelsLike(Double feelsLike) {
            this.feelsLike = feelsLike;
        }

        public Double getTempMin() {
            return tempMin;
        }

        public void setTempMin(Double tempMin) {
            this.tempMin = tempMin;
        }

        public Double getTempMax() {
            return tempMax;
        }

        public void setTempMax(Double tempMax) {
            this.tempMax = tempMax;
        }

        public Integer getPressure() {
            return pressure;
        }

        public void setPressure(Integer pressure) {
            this.pressure = pressure;
        }

        public Integer getHumidity() {
            return humidity;
        }

        public void setHumidity(Integer humidity) {
            this.humidity = humidity;
        }

    }

    public class Sys {

        @SerializedName("type")
        private Integer type;
        @SerializedName("id")
        private Integer id;
        @SerializedName("country")
        private String country;
        @SerializedName("sunrise")
        @Expose
        private PrettyTime sunrise;
        @SerializedName("sunset")
        @Expose
        private PrettyTime sunset;

        public Integer getType() {
            return type;
        }

        public void setType(Integer type) {
            this.type = type;
        }

        public Integer getId() {
            return id;
        }

        public void setId(Integer id) {
            this.id = id;
        }

        public String getCountry() {
            return country;
        }

        public void setCountry(String country) {
            this.country = country;
        }

        public PrettyTime getSunrise() {
            return sunrise;
        }

        public void setSunrise(PrettyTime sunrise) {
            this.sunrise = sunrise;
        }

        public PrettyTime getSunset() {
            return sunset;
        }

        public void setSunset(PrettyTime sunset) {
            this.sunset = sunset;
        }

    }

    public class Weather {

        @SerializedName("id")
        private Integer id;
        @SerializedName("main")
        private String main;
        @SerializedName("description")
        private String description;
        @SerializedName("icon")
        private String icon;

        public Integer getId() {
            return id;
        }

        public void setId(Integer id) {
            this.id = id;
        }

        public String getMain() {
            return main;
        }

        public void setMain(String main) {
            this.main = main;
        }

        public String getDescription() {
            return description;
        }

        public void setDescription(String description) {
            this.description = description;
        }

        public String getIcon() {
            return icon;
        }

        public void setIcon(String icon) {
            this.icon = icon;
        }

    }

    public class Wind {

        @SerializedName("speed")
        private Double speed;
        @SerializedName("deg")
        private Integer deg;
        @SerializedName("gust")
        private Double gust;

        public Double getSpeed() {
            return speed;
        }

        public void setSpeed(Double speed) {
            this.speed = speed;
        }

        public Integer getDeg() {
            return deg;
        }

        public void setDeg(Integer deg) {
            this.deg = deg;
        }

        public Double getGust() {
            return gust;
        }

        public void setGust(Double gust) {
            this.gust = gust;
        }
    }

}

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