Activity/Fragment lifecycle causes NullPointerException

Question

I'm trying to pass data from an activity to a fragment. The onStart method from the activity calls the getUID method inside the fragment and passes a String, the fragment on the other end receives the String and sets the text from a TextView.

The problem is that the fragment is null by the time the activity calls the method in the fragment:

Stack Trace:

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.test.project/com.test.project.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.test.project.Chat.getUID(android.os.Bundle)' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2434) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2494) at android.app.ActivityThread.access$900(ActivityThread.java:153) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1347) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:148) at android.app.ActivityThread.main(ActivityThread.java:5451) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.test.project.Chat.getUID(android.os.Bundle)' on a null object reference at com.test.project.MainActivity.onStart(MainActivity.java:178) at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1238) at android.app.Activity.performStart(Activity.java:6340) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2397) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2494)  at android.app.ActivityThread.access$900(ActivityThread.java:153)  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1347)  at android.os.Handler.dispatchMessage(Handler.java:102)  at android.os.Looper.loop(Looper.java:148)  at android.app.ActivityThread.main(ActivityThread.java:5451)  at java.lang.reflect.Method.invoke(Native Method)  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

MainActivity.java

@Override
protected void onCreate(Bundle savedInstanceState) {

    MyClass myClass = new MyClass();


    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    fragmentManager.beginTransaction().replace(R.id.container, myClass).addToBackStack(null).commit();

    mAuth = FirebaseAuth.getInstance();

}

...

protected void onStart() {
    super.onStart();
    // Check if user is signed in (non-null) and update UI accordingly.
    FirebaseUser currentUser = mAuth.getCurrentUser();
    if (currentUser == null ) {
        ...

    } else {

        String uid = currentUser.getUid();

        Chat chatFragment = (Chat)getSupportFragmentManager().findFragmentById(R.id.chat_layout);
        Bundle bundle = new Bundle();
        bundle.putString("uid", uid);
        chatFragment.getUID(bundle);

This class below creates a ViewPager that populates the tabs in a tabLayout with ViewPagerAdapter.

MyClass.java

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment


    final View rootView = inflater.inflate(R.layout.fragment_my_class, container, false);

    viewPager = (ViewPager) rootView.findViewById(R.id.view_pager);
    viewPager.setAdapter(new ViewPagerAdapter(getActivity().getSupportFragmentManager()));

    tabLayout = (TabLayout) rootView.findViewById(R.id.tab_layout);
    tabLayout.setupWithViewPager(viewPager);

    return rootView;
}

ViewPagerAdapter

public ViewPagerAdapter(FragmentManager fragmentManager) {
    super(fragmentManager);
    Log.e("error","Adapter");
    childFragments = new Fragment[]{
            new Chat(),
            new Reminders(),
            new Agenda()
    };

}

@Override
public Fragment getItem(int position) {
    Log.e("error", String.valueOf(position));
    return childFragments[position];
}

@Override
public int getCount() {

    return childFragments.length;
}

@Override
public CharSequence getPageTitle(int position){

    String title = getItem(position).getClass().getName();
    return title.subSequence(title.lastIndexOf(".") + 1, title.length());
}

Chat.java (fragment)

public class Chat extends Fragment {

private TextView text1;

public void getUID(Bundle uid){
        String gotuid = uid.getString("uid");
        text1.setText(gotuid);
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {

    final View rootView = inflater.inflate(R.layout.fragment_chat, container, false);
    text1 = (TextView) rootView.findViewById(R.id.uidTextView);
    return rootView;
    }
}

I put some Log.i here and there and found out that the Chat.java fragment is created sometime after the MainActivity.java's onStart() is finished. How do I let the activity know when the Fragment is displayed/created so that the activity can send data to it?

Any help would be appreciated.