NullPointerException calling Flow.get(this) in the onCreate method.

I am getting the following error when I call Flow.get(this) inside the onCreate() method.

Caused by: java.lang.NullPointerException: Attempt to read from field 'flow.Flow flow.InternalLifecycleIntegration.flow' on a null object reference
                                                     at flow.InternalContextWrapper.getSystemService(InternalContextWrapper.java:54)
                                                     at android.view.ContextThemeWrapper.getSystemService(ContextThemeWrapper.java:123)
                                                     at android.app.Activity.getSystemService(Activity.java:5263)
                                                     at flow.InternalContextWrapper.getFlow(InternalContextWrapper.java:31)
                                                     at flow.Flow.get(Flow.java:46)
                                                     at flow.sample.basic.BasicSampleActivity.onCreate(BasicSampleActivity.java:30)
                                                     at android.app.Activity.performCreate(Activity.java:6237)
                                                     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
                                                     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
                                                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
                                                     at android.app.ActivityThread.-wrap11(ActivityThread.java) 
                                                     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
                                                     at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                     at android.os.Looper.loop(Looper.java:148) 
                                                     at android.app.ActivityThread.main(ActivityThread.java:5417) 
                                                     at java.lang.reflect.Method.invoke(Native Method)

The code:

public class BasicSampleActivity extends Activity {

  @Override protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.basic_activity_frame);

    Flow flow = Flow.get(this); // <============== ERROR!!! NPE
    System.out.println(flow);
  }

  @Override protected void attachBaseContext(Context baseContext) {
    baseContext = Flow.configure(baseContext, this) //
        .dispatcher(new BasicDispatcher(this)) //
        .defaultKey(new WelcomeScreen()) //
        .keyParceler(new BasicKeyParceler()) //
        .install();
    super.attachBaseContext(baseContext);
  }

  @Override public void onBackPressed() {
    if (!Flow.get(this).goBack()) {
      super.onBackPressed();
    }
  }
}

Somebody can explain me why this is happening in the onCreate() method? If I do the same in onStart() or onResume() everything works well.

I am not sure if this is a bug or not, but I think a wiki/documentation is required for this awesome library.

[Zhuinden]

@epool you might be correct, Flow is initialized in onActivityCreated() method of InternalLifecycleIntegration (retained fragment), so you might be better off if you move that logic to onPostCreate()

[loganj]

+1 to handling this in the dispatcher.