title: AccessToken.getCurrentAccessToken() is null
body: <p>I Integrated Facebook in my app and getting the proper token. If the token already exists then it assigns it, and if it's the first time then it will make a request for the token. However, the token is not being received.</p>
  
  <p>Here is my code: </p>
  
  <pre><code>@Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          FacebookSdk.sdkInitialize(this.getApplicationContext());
          callbackManager = CallbackManager.Factory.create();
  
          accessTokenTracker = new AccessTokenTracker() {
              @Override
              protected void onCurrentAccessTokenChanged(
                      AccessToken oldAccessToken,
                      AccessToken currentAccessToken) {
                  // Set the access token using
                  // currentAccessToken when it's loaded or set.
              }
          };
          // If the access token is available already assign it.
          accessToken = AccessToken.getCurrentAccessToken();
          setContentView(R.layout.activity_signup);
  </code></pre>