QML types can retain invalid references when QQmlEngine is destroyed before the loading is finished

The type loader frequently passes method calls back and forth between its own thread and the main thread. In order to make sure the objects needed by the method calls are not deleted in between, it takes a reference before posting a message to the other thread, and releases that reference once the method has been executed. See for example QQmlTypeLoaderThread::load and QQmlTypeLoaderThread::loadThread to illustrate this. When the QML engine gets deleted, all messages queued by the type loader are dropped. This means the references taken for those messages are never released, and the objects referenced that way can never be deleted anymore.