Don't crash when passive grabber deleted before exclusive gr. released

In this scenario, a DragHandler is inside an Item in a Loader, under
a MouseArea, which unloads the loader on press.  So on press, the
DragHandler acquires a passive grab, then the MouseArea acquires the
exclusive grab, then the DragHandler is destroyed along with its parent
when the Loader is unloaded.  On release,
QQuickEventPoint::setGrabberItem(nullptr) was sending an
onGrabChanged(passiveGrabber, OverrideGrabPassive, this) notification.
That was questionable: the handler was not just then getting its grab
overridden, but rather un-overridden, because the exclusive grab
was being released.  It's also a good idea to check for null pointers,
since m_passiveGrabbers is a collection of QPointers already,
so we can tell when a passive grabber is deleted dynamically.

It can also be reproduced with MultiPointTouchArea just as with
MouseArea, so the test is written that way for convenience, because
we have tst_multipointtoucharea_interop already.  It doesn't really
matter which handler has the passive grab, or which item has the
exclusive grab that's being relinquished.

Fixes: QTBUG-73819
Change-Id: Ic605efa2143a1d849be095dcb88d6c38d7d2ee19
Reviewed-by: Jan Arve Sæther <jan-arve.saether@qt.io>

0 files changed, 0 insertions, 0 deletions