app crash when trying to use picture-in-picture mode

[cosven]

Describe the bug
app crash when trying to use picture-in-picture mode

To Reproduce

1. watch a video and click '画中画' button

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots

Desktop:

• Wayland

possible root cause

Maybe mpv video output should be set to nil before it's QOpenGLWidget changes parent.

FeelUOwn/feeluown/gui/watch.py

Lines 59 to 65 in e7994ff

	@contextmanager
	def change_parent(self):
	self._parent_is_changing = True
	self._before_change_mpv_widget_parent()
	yield
	self._after_change_mpv_widget_parent()
	self._parent_is_changing = False

[cosven]

should be fixed by #798