Gesture handlers crash due to misbehaving compositors

I've got a report named "Random segfaults while switching desktop with gestures" with "Switch a few times to the left/right desktop using three-finger gestures." in reproduction steps on "Arch LInux, GNOME 43.3, Wayland". I haven't reproduce this myself (I have no such environment), but the crash dumps and the WAYLAND_DEBUG log make me think it was introduced with https://github.com/qt/qtwayland/commit/4ba3bd736e6ba1835a0b0b51d101101c39a58433. The code in _update and _end functions assumes mFocus is always set, but if _begin wasn't called (or _end was called previously), it's not set. This is probably a compositor bug, but I believe Qt can do a better job by just ignoring the events if mFocus is not set (or print some warning about compositor bug) rather than crashing.