core: harden the handling of received messages

main.py

@@ -319,12 +319,14 @@ def queue_processor():
 def message_handler(update, context):
     global lock
     with lock:
-        if update.message.text.__class__ == str and update.message.text.startswith("$"):
+        if update.message and update.message.text.__class__ == str and update.message.text.startswith("$"):
             print("[DEBUG] Received new message with high priority")  # just for testing
             message_queue.insert(0, update.message)
-        else:
+        elif update.message:
             print("[DEBUG] Received new message")  # just for testing
             message_queue.append(update.message)
+        else:
+            print(f"[DEBUG] Received {update.message} instead of a message")
 
 
 # --- Final stage ---