why is requestIdToken returning null?

In my case i had previously,

 GoogleSignInOptions gso = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
            .requestEmail()
            .build();

I fixed it by adding adding the requestToken method,

 GoogleSignInOptions gso = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
            .requestEmail()
            .requestIdToken(getString(R.string.default_web_client_id))
            .build();

Now its working for me.Hopefully this can help somebody having issues like me.

As I have answered at the following question:

Google Sign-In requestIdToken returns null

In order to request Id Token sucessfully, you should use a "Web application" type Client Id, instead of "Android" type Client Id.

You also find at Google's documentation, the following info (please note #3):

Create an OAuth 2.0 client ID for your backend server

If your app authenticates with a backend server or accesses Google APIs from your backend server, you must create an OAuth 2.0 client ID for your server. To create an OAuth 2.0 client ID:

1. Open the Credentials page.
2. Click Add credentials > OAuth 2.0 client ID.
3. Select Web application.
4. Click Create.

Pass this client ID to the requestIdToken or requestServerAuthCode method when you create the GoogleSignInOptions object.

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Update Mar 26:

Android Developers Blog - Registering OAuth clients for Google Sign-In