Add try-finally around main loop.

Demo/tkinter/guido/electrons.py

@@ -52,8 +52,11 @@ class Electrons:
 
 	# Run -- never returns
 	def run(self):
+		try:
 			while 1:
 				self.random_move(self.n)
+		finally:
+			self.tk.destroy()
 
 
 # Main program