Abort effect if the current tab box window is deleted and the tabbox is empty afterwards. This hopefully resolves the crashes when a window is closed during active effect. At least I was not able to reproduce a crash anymore. If further testing shows that it resolves the problem the patch will be backported to branch and to other window switching effects showing the same problem.
CCBUG: 182358

svn path=/trunk/KDE/kdebase/workspace/; revision=919075