Android Activity lifecycle problems

Question

I'm working on the project initially created by another developer. There is a root Activity (let's call it CustomActivity) with code below.

private static SomeOtherClass instance = null;

@Override
public void onCreate(final Bundle savedInstanceState)
{
    super.onCreate(null);

    if (!(Thread.getDefaultUncaughtExceptionHandler() instanceof CustomExceptionHandler)) {
        Thread.setDefaultUncaughtExceptionHandler(new CustomExceptionHandler());
    }

    // @see http://stackoverflow.com/questions/19545889/app-restarts-rather-than-resumes)
    if (!isTaskRoot() && getIntent().hasCategory(Intent.CATEGORY_LAUNCHER) && null != getIntent().getAction()
            && getIntent().getAction().equals(Intent.ACTION_MAIN)) {
        finish();
        return;
    }

    instance = new SomeOtherClass();

    this.screen = new CustomFragment();
    this.getFragmentManager.beginTransaction()
      .replace(R.id.content_frame, screen).commit();

    ...
}

@Override
protected void onDestroy()
{
    this.instance = null;
    ...
}

static public SomeOtherClass getInstance()
{
    return instance;
}

this.screen has a button with CustomActivity.getInstance().methodCall() on tap. And one of beta testers said he just tapped that button and got crash with this method in stacktrace: Attempt to invoke virtual method '...CustomActivity.getInstance().methodCall()' on a null object reference.

I don't understand - how it's possible due to Activity lifecycle.

According to stacktrace, onCreate shouldn't be called after some previous onDestroy. Even if the last one can happen when we were in another Activity (and yep, this.screen is not nullified anywhere), but this.screen fragment can't be rendered without Activity recreation. Am I right?

P.S.: there is no more instance variable management at all and SomeOtherClass has no custom parent class (just default object).

P.P.S.: nope, device wasn't locked / app just launched. Tester worked with it, rotated phone to remove sim card, removed, rotated back and saw crash alert.

P.P.P.S: don't know why null in super.onCreate() but anyhow this Activity has no code to support saved states.