handle unknown command

bot.py

@@ -123,6 +123,7 @@ class Bot:
 
         moduleobject = self.modules.get(command)
 
+        if moduleobject is not None:
             if moduleobject.enabled:
                 try:
                     await moduleobject.matrix_message(bot, room, event)
@@ -134,6 +135,10 @@ class Bot:
                     await self.send_text(room,
                                          f'Module {command} experienced difficulty: {sys.exc_info()[0]} - see log for details')
                     traceback.print_exc(file=sys.stderr)
+        else:
+            print(f"Unknown command: {command}")
+            await self.send_text(room, f"Sorry. I don't know what to do. Execute !help to get a list of available commands.")
+
 
     async def invite_cb(self, room, event):
         room: MatrixRoom