Fix traceback when on_change_command is not set

src/wgfrontend/config.py

@@ -124,6 +124,7 @@ class Configuration():
     def on_change_command(self):
         """The command to be executed on config changes"""
         cmd = self.config.get('on_change_command')
+        if cmd is not None:
             cmd = cmd.strip('"\'')
         return cmd