Crash when switching between output nodes with Frame Props dialog.

[JustinTArthur]

QLayout::removeWidget: Cannot remove a null widget.
Traceback (most recent call last):
  File "vspreview/main/window.py", line 557, in switch_output
    self.switch_frame(self.current_output.last_showed_frame)
  File "vspreview/main/window.py", line 530, in switch_frame
    toolbar.on_current_frame_changed(frame)
  File "vspreview/toolbars/main/toolbar.py", line 146, in on_current_frame_changed
    self.frame_props_dialog.update_frame_props(self.main.current_output.props)
  File "vspreview/toolbars/main/dialog.py", line 117, in update_frame_props
    self.framePropsVLayout.clear()
  File "vspreview/core/abstracts.py", line 106, in clear
    widget.setParent(None)  # type: ignore
AttributeError: 'NoneType' object has no attribute 'setParent'