The admob ad native error should be fixed?

native ad mobile opened. This error is causing this error for me. I tried to fix this error several times but I could not fix it. Native ad is causing this error.

Error

*E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.myapplication, PID: 19618
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.myapplication/com.example.myapplication.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.google.android.gms.ads.AdView.setAdSize(com.google.android.gms.ads.AdSize)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3318)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3429)
        at android.app.ActivityThread.-wrap12(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2009)
        at android.os.Handler.dispatchMessage(Handler.java:109)
        at android.os.Looper.loop(Looper.java:166)
        at android.app.ActivityThread.main(ActivityThread.java:7555)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:469)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:963)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.google.android.gms.ads.AdView.setAdSize(com.google.android.gms.ads.AdSize)' on a null object reference
        at com.example.myapplication.MainActivity.loadBanner(MainActivity.java:59)
        at com.example.myapplication.MainActivity.onCreate(MainActivity.java:38)
        at android.app.Activity.performCreate(Activity.java:7343)
        at android.app.Activity.performCreate(Activity.java:7333)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1219)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3271)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3429) 
        at android.app.ActivityThread.-wrap12(Unknown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2009) 
        at android.os.Handler.dispatchMessage(Handler.java:109) 
        at android.os.Looper.loop(Looper.java:166) 
        at android.app.ActivityThread.main(ActivityThread.java:7555) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:469) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:963) 
I/Process: Sending signal. PID: 19618 SIG: 9*

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