secure m_highlightedWindows access when triggering

explicit repaint in highlight effect

effects/highlightwindow/highlightwindow.cpp

@@ -245,6 +245,7 @@ void HighlightWindowEffect::prepareHighlighting()
     foreach( EffectWindow *w, effects->stackingOrder() )
         if( !m_windowOpacity.contains( w )) // Just in case we are still finishing from last time
             m_windowOpacity[w] = isInitiallyHidden( w ) ? 0.0 : 1.0;
+    if (!m_highlightedWindows.isEmpty())
         m_highlightedWindows.at(0)->addRepaintFull();
     }