Crash in delivery of gesture events to disappearing tooltip

QGuiApplication fails to check for a valid receiver before sending gesture events. To reproduce open assistant on a Mac with a trackpad or magic mouse. Hover over one of the toolbar buttons until the tooltip pops up. Move the mouse cursor over the tooltip and scroll as the tooltip starts to disappear.

It's easiest to reproduce with the magic mouse middle scroll gesture, but I can also get it to crash with the trackpad two finger scroll gesture.

I have a patch which I'll propose shortly on gerrit. This issue has plagued me ever since switching to Qt 5. I think that it should be a release blocker.