How come I cannot call a method in my fragment

Question

EDIT: THE ISSUE HERE IS NOT THE NULLPOINTER EXPECTION, the issue is that when I'm trying to send an int to a method located inside one of my fragments, it CRASHES.

I have been searching everywhere for a solution, but I have not yet found one, which is why I created an account and posted. Please do not mark this as duplicate (unless you can find a solution to the issue as to why I am not able to call frag.changeText(id); for this situation). It keeps crashing and I still can't figure out why it cannot call this method.

Here is my Main Fragment Code (inside Main Activity):

public class MainFragment extends Fragment {

    private static final int REQUEST_CODE_DETAILS_ACTIVITY = 1234;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment_main, container, false);
        //return inflater.inflate(R.layout.fragment_screen2, container, false);
    }

    @Override
    public void onActivityCreated(Bundle savedState) {
        super.onActivityCreated(savedState);

        Button b1 = (Button) getActivity().findViewById(R.id.button1);
        b1.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View view) {
                onClickMethod(R.id.button1);
            }
        });

       // Button b2 = (Button) getActivity().findViewById((R.id.button2);
        //b2.setOnClickListener(new View.);
    }

    public void onClickMethod(int id) {
        // int id = R.id.button1;
        Toast.makeText(getActivity(), "does the id get passed?" +id , Toast.LENGTH_SHORT).show();

        if (getResources().getConfiguration().orientation == Configuration.ORIENTATION_LANDSCAPE) {
            int testint = 323;
            FragScreen2 frag = (com.example.engg6600.fragtest2.FragScreen2) getFragmentManager().findFragmentById(R.id.fragment_screen2);
            //frag.changeText(id);
            try {
                ((FragScreen2)getFragmentManager().findFragmentById(R.id.fs2)).testMethod(testint);
            } catch (Exception e) {
                Toast.makeText(getActivity(), "breaks" + testint, Toast.LENGTH_SHORT).show();
            }
            //Intent goS2 = new Intent(getActivity(), Screen2.class);
            //goS2.putExtra("buttonNumber", id);

//            FragScreen2 frag = (FragScreen2) getFragmentManager().findFragmentById(R.id.fragment_screen2);
//            Toast.makeText(getActivity(), "before to frag.changetext(id)", Toast.LENGTH_LONG).show();
//            frag.changeText(id);
//            Toast.makeText(getActivity(), "sent to frag.changetext(id)", Toast.LENGTH_LONG).show();
        } else{

           // String toastString = Integer.toString(id);
            Toast.makeText(getActivity(), "portrait mode id" +id, Toast.LENGTH_LONG).show();
            Intent goS2 = new Intent(getActivity(), Screen2.class);
            goS2.putExtra("buttonNumber", id);
            startActivityForResult(goS2, REQUEST_CODE_DETAILS_ACTIVITY);
        }
    }

}

FragScreen2 Fragment Code:

public class FragScreen2 extends Fragment {

    private static final String[] SCREENTEXT ={
            "You have selected button 1",
            "You have selected button 2",
            "You have selected button 3",
            "You have selected button 4",
            "No selection"

    };

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment_screen2, container, false);
    }

    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);

        // pull out the country ID to show from the activity's intent
        Intent intent = getActivity().getIntent();
        int id = intent.getIntExtra("buttonNumber", 234324);
        Toast.makeText(getActivity(), "fragscreen 2 id=" +id, Toast.LENGTH_LONG).show();
        changeText(id);
    }

    public void changeText(int id) {
        int buttonNumber=4;
        Toast.makeText(getActivity(), "button1 of R.id" + Integer.toString(R.id.button1), Toast.LENGTH_SHORT).show();
        if (id==R.id.button1) {
            buttonNumber =0;
        } else if (id==R.id.button2) {
            buttonNumber =1;
        } else if (id==R.id.button3) {
            buttonNumber =2;
        } else if (id==R.id.button4) {
            buttonNumber =3;
        }
        String text = SCREENTEXT[buttonNumber];
        TextView tv = (TextView)getActivity().findViewById(R.id.textscreen2);
        tv.setText(text);

    }

    public void testMethod (int integerVar) {
        Toast.makeText(getActivity(), "testmethod int=" + integerVar, Toast.LENGTH_SHORT).show();
    }
}

and I keep getting this error (BEFORE I added the try catch method):

10-21 16:04:07.263 25153-25153/com.example.engg6600.fragtest2 W/dalvikvm: threadid=1: thread exiting with uncaught exception (group=0x419a9da0) 10-21 16:04:07.263 25153-25153/com.example.engg6600.fragtest2 E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.engg6600.fragtest2, PID: 25153 java.lang.NullPointerException at com.example.engg6600.fragtest2.MainFragment$override.onClickMethod(MainFragment.java:48) at com.example.engg6600.fragtest2.MainFragment$override.access$dispatch(MainFragment.java) at com.example.engg6600.fragtest2.MainFragment.onClickMethod(MainFragment.java:0) at com.example.engg6600.fragtest2.MainFragment$1.onClick(MainFragment.java:33) at android.view.View.performClick(View.java:4654) at android.view.View$PerformClick.run(View.java:19438) at android.os.Handler.handleCallback(Handler.java:733) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:146) at android.app.ActivityThread.main(ActivityThread.java:5602) at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:515) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1283) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1099) at dalvik.system.NativeStart.main(Native Method)

My question is: why can't I call the function located in my other fragment? or pass the int value to it?