Crash when pressing (un)mute button when there's no audio track

[LightArrowsEXE]

This should probably throw a warning, but not flat-out crash vspreview.

Traceback (most recent call last):
  File "C:\Users\light\AppData\Local\Programs\Python\Python310\lib\site-packages\vspreview\toolbars\playback\toolbar.py", line 530, in on_mute_clicked
    self.setMute(not self.audio_muted)
  File "C:\Users\light\AppData\Local\Programs\Python\Python310\lib\site-packages\vspreview\toolbars\playback\toolbar.py", line 537, in setMute   
    self.play_audio()
  File "C:\Users\light\AppData\Local\Programs\Python\Python310\lib\site-packages\vspreview\toolbars\playback\toolbar.py", line 259, in play_audio
    self.current_audio_frame = self.current_audio_output.to_frame(
AttributeError: 'NoneType' object has no attribute 'to_frame'