Home Assistant reports an error when opening and closing the xiaomi curtain

I’m newbie and use Zigbee2mqtt for 2 days. When using it, I discovered that every time the curtain is closing or opening, Home Assistant system will issue a warning like the image below.

In the process of understanding, I learned that the mqtt command for the curtain only accepts the position in the form of numbers. But our system send the null value so it causes an error for the mqtt system. I tried to set retain = true, but it didn’t seem to be useful for this case.
System used:
  • Home Assistant version 0.97.2, Hass.io supervisor version 180
  • Zigbee2mqtt Hass.io add-on (version 1.5.1)
  • Coordinator firmware version: ‘20190223’
    Thanks for reading and give me advice.