DeprecationWarning: Unhandled promise rejections are deprecated for RF Bridge
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### What issue do you have? Please be as thorough and explicit as possible.
There is a TypeError in the homebridge log [rf-bridge.js:164:51](https://github.com/bwp91/homebridge-ewelink/blob/latest/lib/device/rf-bridge.js#L164).
It appears after upgrading Homebridge to v1.3.8 from v1.3.6. 
There are alerts configured on RF433 and so once they triggered we can see errors in the log file.

### Details of your setup.
* Do you use (1) Homebridge UI-X (2) Homebridge CLI or (3) HOOBS? 
(1)


* Which version of Homebridge/HOOBS do you have?
v1.3.8


* Which platform do you run Homebridge/HOOBS on (e.g. Raspberry Pi/Windows/HOOBS Box)? Please also mention your version of Node.js/NPM if known.
Raspberry Pi
npm: v8.1.4
node.js: v14.18.1


* Which version of this plugin (homebridge-ewelink) do you have? Has the issue started since upgrading from a previous version?
v8.6.0


* Which eWeLink devices do you have that are causing issues? Please include product models if applicable.
Sonoff RF Bridge RFBridge433
fw: 3.5.0


### Please paste any relevant logs below.
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```
[eWeLink] Initialising plugin v8.6.0 | Node v14.18.1 | HB v1.3.8...
[23.11.2021, 09:43:31] [eWeLink] [Свет] current state [button triggered].
[23.11.2021, 09:43:31] [eWeLink] [Свет] current state [button triggered].
[23.11.2021, 09:43:31] [eWeLink] [Кнопка Коридор] current state [triggered].
(node:27195) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'updateCharacteristic' of undefined
    at /usr/lib/node_modules/homebridge-ewelink/lib/device/rf-bridge.js:164:51
(Use `node --trace-warnings ...` to show where the warning was created)
(node:27195) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 7)
(node:27195) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
[23.11.2021, 09:44:31] [eWeLink] [Кнопка Коридор] current state [not triggered].
```
