QGraphicsProxyWidget::setWidget() is checking if the newly assigned
widget is already assigned to a child proxy widget without checking if
the child has a widget assigned at all which lead to a nullptr reference
if it is not the case.
Therefore check if the assigned widget is a valid pointer.
Fixes: QTBUG-15442
Change-Id: I006877f99895ca01975bdcad071cfcf90bea22ad
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
794140fb86