exception when trying to remove last button

[pmaanen]

When trying to remove the last button, the following exception is thrown:
Exception in Tkinter callback
Traceback (most recent call last):
File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/lib-tk/Tkinter.py", line 1536, in call
return self.func(*args)
File "soundboard.py", line 20, in onRightClick
self.parent.master.myButtons.remove(self)
ValueError: list.remove(x): x not in list