Attempt to invoke virtual method 'void android.support.v4.widget.DrawerLayout.setDrawerShadow(int, int)' on a null object reference

Due to my lack of knowledge on this I'm jumping from one problem to another. I've been trying to figure it out for hours now and looked through various previous questions but not getting anywhere. Most recent is this error:

 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v4.widget.DrawerLayout.setDrawerShadow(int, int)' on a null object reference
        at XXXX.NavigationDrawerFragment.setUp(NavigationDrawerFragment.java:136)
        at XXXX.EditFactFind.onCreate(EditFactFind.java:72)
        at android.app.Activity.performCreate(Activity.java:5990)

My Activity (the main point that it's failing) is:

import java.io.Serializable;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Calendar;


import android.app.ActionBar;
import android.app.Activity;
import android.app.AlertDialog;
import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.content.DialogInterface;
import android.content.Intent;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.widget.DrawerLayout;
//import android.support.v7.app.ActionBar;
import android.support.v7.app.AppCompatActivity;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class EditFactFind extends Activity implements NavigationDrawerFragment.NavigationDrawerCallbacks {

    public static final int RESULT_DELETE = -500;
    private boolean isInEditMode = true;
    private boolean isAddingFactFind = true;
    private NavigationDrawerFragment mNavigationDrawerFragment;
    private CharSequence mTitle;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.edit_factfind);

        final Button saveButton = (Button)findViewById(R.id.saveButton);
        final Button cancelButton = (Button)findViewById(R.id.cancelButton);
        final EditText titleEditText = (EditText)findViewById(R.id.titleEditText);
        //final EditText factFindEditText = (EditText)findViewById(R.id.factFindEditText);
        final TextView dateTextView = (TextView)findViewById(R.id.dateTextView);
        final Button nextButton =(Button) findViewById(R.id.nextButton);

        //Create fragment and give it an argument for the selected article
        secA_pg1 iniSecFrag = new secA_pg1();
        Bundle args = new Bundle();
        args.putInt(secA_pg1.ARG_INDEX, 1);
        iniSecFrag.setArguments(args);

        FragmentTransaction initialTransaction = getFragmentManager().beginTransaction();

        // Replace whatever is in the fragment_container view with this fragment,
        // and add the transaction to the back stack so the user can navigate back
        initialTransaction.replace(R.id.fragment_container, iniSecFrag);
        initialTransaction.addToBackStack(null);

        //Commit the transaction
        initialTransaction.commit();

        mNavigationDrawerFragment = (NavigationDrawerFragment)
                getFragmentManager().findFragmentById(R.id.navigation_drawer);
        mTitle = getTitle();

        // Set up the drawer.
        mNavigationDrawerFragment.setUp(
                R.id.navigation_drawer,
                (DrawerLayout) findViewById(R.id.drawer_layout));

        Serializable extra = getIntent().getSerializableExtra("FactFind");
        if(extra != null)
        {
            FactFind factFind = (FactFind) extra;
            titleEditText.setText(factFind.getTitle());
          //  factFindEditText.setText(factFind.getFactFindTitle());

            DateFormat dateFormat = new SimpleDateFormat("dd-MM-yyyy HH:mm:ss");
            String date = dateFormat.format(factFind.getDate());

            dateTextView.setText(date);

            isInEditMode = false;
            titleEditText.setEnabled(false);
          //  factFindEditText.setEnabled(false);
            saveButton.setText("Edit");

            isAddingFactFind = false;

        }

        cancelButton.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                setResult(RESULT_CANCELED, new Intent());
                finish();
            }
        });

        nextButton.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                //secA_pg1 secFrag = (secA_pg1) getFragmentManager().findFragmentById(R.id.Sec_A_pg1_fragment);
                //Create fragment and give it an argument for the selected article
                secA_pg2 newSecFrag = new secA_pg2();
                Bundle args = new Bundle();
                args.putInt(secA_pg2.ARG_INDEX, 2);
                newSecFrag.setArguments(args);

                FragmentTransaction transaction = getFragmentManager().beginTransaction();

                // Replace whatever is in the fragment_container view with this fragment,
                // and add the transaction to the back stack so the user can navigate back
                transaction.replace(R.id.fragment_container, newSecFrag);
                transaction.addToBackStack(null);

                //Commit the transaction
                transaction.commit();
            }
        });

        saveButton.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {


                if(isInEditMode)
                {
                    Intent returnIntent = new Intent();
                    FactFind factFind = new FactFind(titleEditText.getText().toString(),Calendar.getInstance().getTime());
                    returnIntent.putExtra("FactFind", factFind);
                    setResult(RESULT_OK, returnIntent);
                    finish();

                }
                else
                {
                    isInEditMode = true;
                    saveButton.setText("Save");
                    titleEditText.setEnabled(true);
                   // factFindEditText.setEnabled(true);
                }

            }
        });
    }

and my Navigation drawer is failing at the setUp method:

package XXXX;

import android.support.v4.app.FragmentActivity;

import android.app.ActionBar;

//import android.support.v7.app.ActionBar;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.AppCompatActivity;
import android.app.Activity;
import android.app.Fragment;
import android.support.v4.app.ActionBarDrawerToggle;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.content.SharedPreferences;
import android.content.res.Configuration;
import android.os.Bundle;
import android.preference.PreferenceManager;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;
import android.support.v7.widget.Toolbar;

    public void setUp(int fragmentId, DrawerLayout drawerLayout) {
    mFragmentContainerView = getActivity().findViewById(fragmentId);
    mDrawerLayout = drawerLayout;

    // set a custom shadow that overlays the main content when the drawer opens
   // mDrawerLayout.setDrawerShadow(R.drawable.drawer_shadow, GravityCompat.START);
    // set up the drawer's list view with items and click listener

    ActionBar actionBar = getActionBar();
    actionBar.setDisplayHomeAsUpEnabled(true);
    actionBar.setHomeButtonEnabled(true);

    // ActionBarDrawerToggle ties together the the proper interactions
    // between the navigation drawer and the action bar app icon.
    mDrawerToggle = new ActionBarDrawerToggle(
            getActivity(),                    /* host Activity */
            mDrawerLayout,                    /* DrawerLayout object */
            R.drawable.ic_drawer,             /* nav drawer image to replace 'Up' caret */
            R.string.navigation_drawer_open,  /* "open drawer" description for accessibility */
            R.string.navigation_drawer_close  /* "close drawer" description for accessibility */
    ) {
        @Override
        public void onDrawerClosed(View drawerView) {
            super.onDrawerClosed(drawerView);
            if (!isAdded()) {
                return;
            }

        //    getActivity().InvalidateOptionsMenu(); // calls onPrepareOptionsMenu()
        }

        @Override
        public void onDrawerOpened(View drawerView) {
            super.onDrawerOpened(drawerView);
            if (!isAdded()) {
                return;
            }

            if (!mUserLearnedDrawer) {
                // The user manually opened the drawer; store this flag to prevent auto-showing
                // the navigation drawer automatically in the future.
                mUserLearnedDrawer = true;
                SharedPreferences sp = PreferenceManager
                        .getDefaultSharedPreferences(getActivity());
                sp.edit().putBoolean(PREF_USER_LEARNED_DRAWER, true).apply();
            }

          //  getActivity().InvalidateOptionsMenu(); // calls onPrepareOptionsMenu()
        }
    };

    // If the user hasn't 'learned' about the drawer, open it to introduce them to the drawer,
    // per the navigation drawer design guidelines.
    if (!mUserLearnedDrawer && !mFromSavedInstanceState) {
        mDrawerLayout.openDrawer(mFragmentContainerView);
    }

    // Defer code dependent on restoration of previous instance state.
    mDrawerLayout.post(new Runnable() {
        @Override
        public void run() {
            mDrawerToggle.syncState();
        }
    });

    mDrawerLayout.setDrawerListener(mDrawerToggle);
}

XML of the drawer is

<!-- A DrawerLayout is intended to be used as the top-level content view using match_parent for both width and height to consume the full space available. -->
<android.DrawerLayout
    xmlns:android="http://ift.tt/nIICcg"
    xmlns:tools="http://ift.tt/LrGmb4"
    android:id="@+id/drawer_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="XXXX.MainActivity">

    <!-- As the main content view, the view below consumes the entire
         space available using match_parent in both dimensions. -->
    <FrameLayout
        android:id="@+id/container"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

    <!-- android:layout_gravity="start" tells DrawerLayout to treat
         this as a sliding drawer on the left side for left-to-right
         languages and on the right side for right-to-left languages.
         If you're not building against API 17 or higher, use
         android:layout_gravity="left" instead. -->
    <!-- The drawer is given a fixed width in dp and extends the full height of
         the container. -->
    <fragment
        android:id="@+id/navigation_drawer"
        android:layout_width="@dimen/navigation_drawer_width"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        android:name="XXXX.NavigationDrawerFragment"
        tools:layout="@layout/fragment_navigation_drawer" />

</android.DrawerLayout>

I'm assuming that mDrawerLayout isn't being picked up correctly and is therefore returning null but I can't figure out why that is.

Thanks.

via Chebli Mohamed