The dockwidget's toplevel window would be a parent of the container's
window when floating. When plugged back into the mainwindow the
dockwidget's window is destroyed and the container's window along
with it. Added a function toplevelAboutToBeDestroyed to unparent
the containers window before this happens so parentWasChanged will
work correctly.
Change-Id: I06679cfb3a8fa3834c0db0be5973c012b8277275
Reviewed-by: Ulf Hermann <ulf.hermann@digia.com>
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>
54b8c27e03