don't change cursor if adapter is null

Fixes #2970
Closes #2977

// FREEBIE

src/org/thoughtcrime/securesms/ConversationFragment.java

@@ -283,13 +283,17 @@ public class ConversationFragment extends ListFragment
 
   @Override
   public void onLoadFinished(Loader<Cursor> arg0, Cursor cursor) {
+    if (getListAdapter() != null) {
       ((CursorAdapter) getListAdapter()).changeCursor(cursor);
     }
+  }
 
   @Override
   public void onLoaderReset(Loader<Cursor> arg0) {
+    if (getListAdapter() != null) {
       ((CursorAdapter) getListAdapter()).changeCursor(null);
     }
+  }
 
   public interface ConversationFragmentListener {
     public void setComposeText(String text);