Not able to get public channels

I am creating a small app using javascript SDK provided by twilio programmable chat , And I want to get the list of all the channels , To achieve this I am using getPublicChannelDescriptors() method.

below is the code:

$.getJSON(
    '/getToken?identity=' + identity + '&endpointId=' + endpointId,

    function (data) {

        console.log(data);
        chatClient = new Twilio.Chat.Client(data.token);

    chatClient.getPublicChannelDescriptors().then(function(channels) {

        //do something
    });
   }
 );

the error I am getting is below:

twilio-chat.min.js:149 Uncaught TypeError: Cannot read property 'getChannels' of null

I am getting data in response of ajax call and able to create chatClient succesfuly and also I am able to fetch all the channels subscribed by the user by using method getSubscribedChannels() but not able to get public channels.

Any help will be appreciated.

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