Prevent crashing when a window is closed.

CCMAIL: <lmurray@undefinedfire.com>

svn path=/trunk/KDE/kdebase/workspace/; revision=863593

effects/presentwindows.cpp

@@ -151,6 +151,8 @@ void PresentWindowsEffect::postPaintScreen()
     // Update windows that are changing brightness or opacity
     foreach( EffectWindow *w, m_windowData.keys() )
         {
+        if( !w )
+            continue;
         if( m_windowData[w].opacity > 0.0 && m_windowData[w].opacity < 1.0 )
             w->addRepaintFull();
         if( m_windowData[w].highlight > 0.0 && m_windowData[w].highlight < 1.0 )