title: NullPointerException on setDrawerListener
body: <p>First attempt at using the Navigation Drawer. Didn't quite work out.
  During debugging, it crashes at the <strong>setDrawerListener</strong> line.
  I'll add the XMLs if needed.</p>
  
  <pre><code>    super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_categories);
  
      toolbar = (Toolbar) findViewById(R.id.tool_bar);
      setSupportActionBar(toolbar);
  
      mRecyclerView = (RecyclerView) findViewById(R.id.sliderList);
      mRecyclerView.setHasFixedSize(true);
  
      mAdapter = new MyAdapter(slider_titles, silder_icons);
      mRecyclerView.setAdapter(mAdapter);
      mRecyclerView.setLayoutManager(mLayoutManager);
  
      Drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
      mDrawerToggle = new ActionBarDrawerToggle(this, Drawer, toolbar, R.string.drawer_open, R.string.drawer_close)
      {
          @Override
          public void onDrawerOpened(View drawerView)
          {
              super.onDrawerOpened(drawerView);
              // code here will execute once the drawer is opened( As I dont want anything happened whe drawer is
              // open I am not going to put anything here)
          }
  
          @Override
          public void onDrawerClosed(View drawerView)
          {
              super.onDrawerClosed(drawerView);
              // Code here will execute once drawer is closed
          }
      };
  
      Drawer.setDrawerListener(mDrawerToggle);
      mDrawerToggle.syncState();
  </code></pre>
  
  <p>This is the main activity XML. It contains the drawer, and another RecyclerView
  <strong>activity_categories.xml</strong></p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;android.support.v4.widget.DrawerLayout
  xmlns:android="http://schemas.android.com/apk/res/android"
  android:id="@+id/drawer_layout"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:elevation="7dp"&gt;
  
  &lt;LinearLayout
      android:orientation="vertical"
      android:layout_width="match_parent"
      android:layout_height="match_parent"&gt;
  
      &lt;include
          android:id="@+id/tool_bar"
          layout="@layout/tool_bar"&gt;
      &lt;/include&gt;
  
      &lt;android.support.v7.widget.RecyclerView
          android:id="@+id/categoryList"
          android:layout_width="match_parent"
          android:layout_height="match_parent"/&gt;
  
  &lt;/LinearLayout&gt;
  
  &lt;android.support.v7.widget.RecyclerView
      android:id="@+id/sliderList"
      android:layout_width="320dp"
      android:layout_height="match_parent"
      android:layout_gravity="left"
      android:background="#ffffff"
      android:scrollbars="vertical"&gt;
  
  &lt;/android.support.v7.widget.RecyclerView&gt;
  &lt;/android.support.v4.widget.DrawerLayout&gt;
  </code></pre>
  
  <p><strong>Log</strong></p>
  
  <pre><code>java.lang.NullPointerException
          at android.support.v7.widget.RecyclerView.onMeasure(RecyclerView.java:1764)
          at android.view.View.measure(View.java:16537)
          at android.support.v4.widget.DrawerLayout.onMeasure(DrawerLayout.java:866)
          at android.view.View.measure(View.java:16537)
          at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5137)
          at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
          at android.view.View.measure(View.java:16537)
          at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5137)
          at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1404)
          at android.widget.LinearLayout.measureVertical(LinearLayout.java:695)
          at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)
          at android.view.View.measure(View.java:16537)
          at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5137)
          at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
          at android.view.View.measure(View.java:16537)
          at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5137)
          at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1404)
          at android.widget.LinearLayout.measureVertical(LinearLayout.java:695)
          at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)
          at android.view.View.measure(View.java:16537)
          at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5137)
          at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
          at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2291)
          at android.view.View.measure(View.java:16537)
          at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1942)
          at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1132)
          at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1321)
          at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1019)
          at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5725)
          at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761)
          at android.view.Choreographer.doCallbacks(Choreographer.java:574)
          at android.view.Choreographer.doFrame(Choreographer.java:544)
          at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747)
          at android.os.Handler.handleCallback(Handler.java:733)
          at android.os.Handler.dispatchMessage(Handler.java:95)
          at android.os.Looper.loop(Looper.java:136)
          at android.app.ActivityThread.main(ActivityThread.java:5086)
          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:785)
          at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
          at dalvik.system.NativeStart.main(Native Method)
  </code></pre>