QWidget::mapTo() crashed!

My application is a video player, and the underlying playback module occasionally reports some information, which is displayed to the user through a bubble notification. This bubble notification is implemented using QWidget, and its parent window is the main window of the application. At the same time, I set some window properties for the notification widget, specifically:
setWindowFlags(windowFlags() | Qt::FramelessWindowHint | Qt::Tool);
During testing, we found that sometimes the bubble notification widget crashes when flush() is called.