Failed getting the cleaning status
Hi. Looks like the new firmware 3.6.8_0.862 on Roborock S5 Max causes errors.

https://github.com/homebridge-xiaomi-roborock-vacuum/homebridge-xiaomi-roborock-vacuum/blob/22e6bd5bc7d757c29470599b5c48462c66156905/index.js#L800

```
homebridge    | [7/10/2020, 4:22:45 PM] [Roborock EG] ERR getCleaning | Failed getting the cleaning status. TypeError: Cannot read property 'property' of null
homebridge    |     at XiaomiRoborockVacuum.get isCleaning [as isCleaning] (/homebridge/node_modules/homebridge-xiaomi-roborock-vacuum/index.js:800:32)
homebridge    |     at XiaomiRoborockVacuum.getCleaning (/homebridge/node_modules/homebridge-xiaomi-roborock-vacuum/index.js:806:31)
homebridge    |     at /homebridge/node_modules/homebridge-xiaomi-roborock-vacuum/index.js:153:49
homebridge    |     at module.exports (/homebridge/node_modules/homebridge-xiaomi-roborock-vacuum/lib/callbackify.js:5:26)
homebridge    |     at On.<anonymous> (/homebridge/node_modules/homebridge-xiaomi-roborock-vacuum/index.js:153:26)
homebridge    |     at On.emit (events.js:315:20)
homebridge    |     at On.EventEmitter.emit (/usr/local/lib/node_modules/homebridge/node_modules/hap-nodejs/src/lib/EventEmitter.ts:42:22)
homebridge    |     at On.Characteristic._this.getValue (/usr/local/lib/node_modules/homebridge/node_modules/hap-nodejs/src/lib/Characteristic.ts:488:12)
homebridge    |     at /usr/local/lib/node_modules/homebridge/node_modules/hap-nodejs/src/lib/Accessory.ts:1215:22
homebridge    |     at Array.forEach (<anonymous>)
homebridge    |     at Bridge.Accessory._this._handleGetCharacteristics (/usr/local/lib/node_modules/homebridge/node_modules/hap-nodejs/src/lib/Accessory.ts:1143:10)homebridge    |     at HAPServer.emit (events.js:315:20)
homebridge    |     at HAPServer.EventEmitter.emit (/usr/local/lib/node_modules/homebridge/node_modules/hap-nodejs/src/lib/EventEmitter.ts:42:22)
homebridge    |     at HAPServer._this._handleCharacteristics (/usr/local/lib/node_modules/homebridge/node_modules/hap-nodejs/src/lib/HAPServer.ts:756:12)
homebridge    |     at IncomingMessage.<anonymous> (/usr/local/lib/node_modules/homebridge/node_modules/hap-nodejs/src/lib/HAPServer.ts:280:24)
homebridge    |     at IncomingMessage.emit (events.js:315:20)
homebridge    |     at endReadableNT (_stream_readable.js:1221:12)
homebridge    |     at processTicksAndRejections (internal/process/task_queues.js:84:21)
```