[REG 5.14.2 -> 5.15.0] Call QQmlIncubator::clear() inside QQmlIncubator::setInitialState() crashes afterward

If a subclass of QQmlIncubator decides that it doesn't want this object anymore during the call of "setInitialState()" and call "clear()", QQmlIncubator will crash after returning from "setInitialState()".

Steps to reproduce:

1. Download the attached test case. Compile and run.
2. Click the button "Load something for nothing.".

As QQmlIncubator::clear()'s documentation doesn't specify any condition which this should not be called, one might assume that it's safe to do so. If it's otherwise not safe to do so, the function should be documented as such.

The stacktrace is obtained from Qt 6.0.1, official binary. The verbose stack is attached, but the relevant frames seems to be:

1  QQmlIncubatorPrivate::incubate               qqmlincubator.cpp         334  0x7ffff7b068e4 
2  QQmlIncubationController::incubateFor        qintrusivelist_p.h        216  0x7ffff7b071e6 
3  QQuickWindowIncubationController::incubate   qquickwindow.cpp          178  0x7fffe8308107 
4  QQuickWindowIncubationController::timerEvent qquickwindow.cpp          161  0x7fffe8308107 

The relevant commit seems to be this:
https://github.com/qt/qtdeclarative/commit/2f3b4ec528f48747a3b7e91e9a7254c25ce24c99#diff-d91d7ca6d9f70fc8304f503b8dd34b7644ffb7674051f2aa42d7784239ba7d04R332-R337

The problem is first discovered on Debian's distribution of Qt 5.15.2, before subsequently confirmed on Qt 5.15.0 and Qt 6.0.1, and verified not to happen on 5.14.2.