Call Fragment Dialog from Activity

Question

I call "DialogFragment" in "fragment" which invokes "FloatingActionButton" in "Activity" and "CallBack interface". But in "Fragment" which calls "DialogFragment" returns the Null value for "GetParentFragmentManager"

In "MainActivity" there is a "ViewPager" and a "FloatingActionButton (FAB)". In "ViewPager" there is a "Fragment" and in it there is "ResyclerView". I'm trying to call "DialogFragment" from FAB. But "getParentFragmentManager" returns NULL.

"floatingActionButton in Main Activity"

`   protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Repository.getInstance().defaultUser();
        mUserName_TV = findViewById(R.id.TV_UserName);
        mUserName_TV.setText(Repository.getInstance().getCurrentUser().getUserName());
        mViewPager2 = findViewById(R.id.viewPager);
        mViewPager2.setAdapter(new ViewPagerAdapter(this));
        mTabLayout = findViewById(R.id.tabLayout);
        mFAB_BTN = findViewById(R.id.fabButton);
        mFAB_BTN.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                mCallBack = new RecyclerFragment();
                mCallBack.createTask(mTabLayout.getSelectedTabPosition());
            }
        });`

"CallBack Method in Fragment"

       @Override
    public void createTask(int tabSelected) {
        NewTaskDialogFragment fragment = NewTaskDialogFragment.newInstance(tabSelected);
        fragment.setTargetFragment(this,REQUEST_CODE_ALTER_DIALOG);
        fragment.show(getFragmentManager(),TAG_DIALOG);
    }
}
interface CallBack
{
    public void createTask(int TabSelected);
}

"DialogFragment"

public Dialog onCreateDialog(Bundle savedInstanceState) { View view = getLayoutInflater().inflate(R.layout.fragment_alter_dialog, null);

initUi(view);
initInfo();
return new AlertDialog.Builder(getActivity())
        .setView(view)
        .setTitle("Create New Task")
        .create();

}