Fix NPE in Launcher onDestroy

Bug: 11765296
Change-Id: I83299656cc90b8a744afb0db4b5ea08675c8db82

src/com/android/launcher3/Launcher.java

@@ -1877,7 +1877,7 @@ public class Launcher extends Activity
 
         mDragLayer.clearAllResizeFrames();
         ((ViewGroup) mWorkspace.getParent()).removeAllViews();
-        mWorkspace.removeAllViews();
+        mWorkspace.removeAllWorkspaceScreens();
         mWorkspace = null;
         mDragController = null;