Qt4: Fix a fullscreen crash

Close #5639
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
(cherry picked from commit c0c2f1c3)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/qt4/main_interface.cpp

@@ -691,7 +691,7 @@ void MainInterface::setVideoFullScreen( bool fs )
         }
 
         /* */
-        if( playlistWidget->artContainer->currentWidget() == videoWidget )
+        if( playlistWidget != NULL && playlistWidget->artContainer->currentWidget() == videoWidget )
         {
             showTab( videoWidget );
         }