title: requestIdToken in GoogleSignInOptions gets null pointer exception
body: <p>I'm trying to add google sign in to my app using these instructions: 
  <a href="https://developers.google.com/identity/sign-in/android/start-integrating" rel="nofollow noreferrer">https://developers.google.com/identity/sign-in/android/start-integrating</a></p>
  
  <p>In my signInActivity I'm getting a null pointer exception on requestIdToken()
  in the command</p>
  
  <pre><code>mSignInOpt = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
                       .requestIdToken(getString(R.string.default_web_client_id))
                       .build();
  </code></pre>
  
  <p>When I use this command without requestIdToken(), it is working perfectly and I'm getting user's name,email etc. in onActivityResult callback, but the token is null.</p>
  
  <p>I made sure that both OAuth 2.0 client IDs in my web console match the ones I use in my app:</p>
  
  <p><a href="https://i.stack.imgur.com/2PcJl.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/2PcJl.png" alt="OAuth 2.0 client IDs in my web console"></a> </p>
  
  <p>I also downloaded google-services.json that the new changes and copied it to my "mobile" folder of the project.</p>
  
  <p><a href="https://i.stack.imgur.com/N34D9.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/N34D9.png" alt="google-services.json I downloaded from firebase console"></a></p>
  
  <p>I've tried all the answers in the links below but nothing solved my problem. </p>
  
  <p>Please Help!</p>
  
  <p>Links:</p>
  
  <ol>
  <li><a href="https://stackoverflow.com/questions/34099208/google-sign-in-requestidtoken-returns-null">Google Sign-In requestIdToken returns null</a></li>
  <li><a href="https://stackoverflow.com/questions/33583326/new-google-sign-in-android">New Google sign in Android</a></li>
  <li><a href="https://stackoverflow.com/questions/33846801/error-12501-authenticating-with-google-sign-in">Error 12501 authenticating with google sign-in</a></li>
  </ol>