Docking/undocking may crash

To reproduce, keep docking and undocking widgets. Easiest way to reproduce seems to be to tabify some docks in a floating window, then docking them back to the main window one by one. I haven't been able to figure out exact pattern.

When it has occurred, there is a crash in QDockAreaLayoutInfo::reparentWidgets where a widgetItem pointer in item_list is 0x60, so it gets past the nullpointer check and crashes. Sometimes it seems to have -1 there as well.