Qt4: Account for rare case when video loads too quickly

Close #6481
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
(cherry picked from commit 9340c4daa66dc15d40eec98535977338e150c5b1)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/qt4/main_interface.cpp

@@ -553,7 +553,9 @@ inline void MainInterface::showTab( QWidget *widget )
         /* Embedded playlist -> Non-embedded playlist */
         if( bgWidget == stackCentralOldWidget && widget == videoWidget )
         {
-            playlistWidget->artContainer->removeWidget( videoWidget );
+            /* In rare case when video is started before the interface */
+            if( playlistWidget != NULL )
+                playlistWidget->artContainer->removeWidget( videoWidget );
             videoWidget->show(); videoWidget->raise();
             stackCentralW->addWidget( videoWidget );
             stackCentralW->setCurrentWidget( videoWidget );