Promise Errors with switches/lights and RF Bridge
**What issue do you have? Please be as thorough and explicit as possible.**

After updating to 3.1.4 when I change the state of Sonoff T1 switches it generates a lot of JS errors related to Promises.

**Details of your setup.**
Homebridge

Which version of Homebridge/HOOBS do you have?

Homebridge 1.2.3

Which version of this plugin (homebridge-ewelink) do you have?

3.1.4

**Please paste any relevant logs below. It helps if you can turn `debug` and `debugReqRes` in the package settings for more thorough logging.**
> If you are posting an error then it is helpful for me to also see the previous few lines as this can show the cause of the error.

```
(node:542) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'externalMultiLightUpdate' of undefined
    at eWeLink.refreshAccessory (/usr/local/lib/node_modules/homebridge-ewelink/lib/eWeLink.js:926:29)
    at eWeLink.receiveDeviceUpdate (/usr/local/lib/node_modules/homebridge-ewelink/lib/eWeLink.js:1035:17)
    at EventEmitter.<anonymous> (/usr/local/lib/node_modules/homebridge-ewelink/lib/eWeLink.js:96:48)
    at EventEmitter.emit (events.js:315:20)
    at /usr/local/lib/node_modules/homebridge-ewelink/lib/eWeLinkLAN.js:101:28
    at Array.forEach (<anonymous>)
    at DnsSd.dns.ondata (/usr/local/lib/node_modules/homebridge-ewelink/lib/eWeLinkLAN.js:53:12)
    at DnsSd._receivePacket (/usr/local/lib/node_modules/homebridge-ewelink/node_modules/node-dns-sd/lib/dns-sd.js:567:9)
    at Socket.<anonymous> (/usr/local/lib/node_modules/homebridge-ewelink/node_modules/node-dns-sd/lib/dns-sd.js:516:9)
    at Socket.emit (events.js:315:20)
(node:542) 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: 3)

```