Fix a bug with GUI's ChannelList.clear not clearing the channel map

gui.py

@@ -695,6 +695,7 @@ class ChannelList:
     def clear(self):
         iids = self._table.get_children()
         self._table.delete(*iids)
+        self._channel_map.clear()
 
     def display(self, channel: Channel, *, add: bool = False):
         # priority