[Q] referencing a dynamica fragment - Android Q&A, Help & Troubleshooting

Hi
Frag1 which is a side navigation drawer, is instantiated in MainActivity.java onCreate by manager.findFragmentById since there is a fragment tag in a xml file.
Frag2 and Frag3 are dynamically committed at some point by a manager, non of their layouts contain a <fragment> tag, they are not committed in the MainActivity onCreate method and thus don't have ids or tags either.
When a button on frag2 is pressed, frag3 "a dialogFragment" needs to be displayed on the screen for user login inputs.
How can I get this to work? I am getting null pointer when referencing frag2 in MainActivity.
here is a illustrative stripped down version of the code. sorry for any unintended typos
Thank you for helping
Frag2.java
Code:
public class Frag2 extends Fragment {
Frag2Communicator = mFrag2communicator;
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
return inflater.inflate(R.layout.frag2_layout, container, false);
class ClickHandler implements View.OnClickListener {
@Override
public void onClick(View v) {
mFrag2communicator.show_frag3();
}
}
}
public interface Frag2Communicator {
public void show_frag3();
}
public void setFrag2Communicator(Frag2Communicator frag2communicator) {
mFrag2communicator = frag2communicator;
}
}
MainActivity.java
Code:
public class MainActivity extends ActionBarActivity implements frag2.frag2Communicator {
protected void onCreate(Bundle savedInstanceState) {
Frag2 frag2 = getSupportFragmentManager().findFragmentById(R.id.frag_2);
//frag_2 is the id of a RelativeLayout inside a ScrollView inside frag2_layout.xml
//I tried to move this ide to the root tag "ScrollView" but still get a null pointer.
frag2.setFrag2Communicator(this);
public void show_frag3(){
FragmentManager manager = getSupportFragmentManager();
Frag3 frag_3 = new Frag3();
frag_3.show(manager, "loging");
}
}
}