Unsubscribing during northbound MQTT data transmission causes a segmentation fault

<img width="748" height="185" alt="Image" src="https://github.com/user-attachments/assets/4351f772-df17-4efc-9cca-ca6f0527970d" />

<img width="719" height="187" alt="Image" src="https://github.com/user-attachments/assets/091249f8-42a4-44f1-9b1d-b5a853d0ad64" />


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*This issue was automatically translated from Chinese.*
