QPropertyAlias crashes in all situations

Since 6.2, QPropertyAlias can no longer be instantiated without leading to immediate crash.

The crash is cased by QPropertyObserver::aliasedProperty() returning nullptr. nullptr is not allowed for setting observer (QBindableInterface::SetObserver).

QPropertyAlias uses QPropertyObserver::aliasedProperty() in all its functions to retrieve the aliased property and strictly requires a pointer to an existing property.

Code example:

QProperty<int> property;
QPropertyAlias<int> alias(&property); // crash