[Bug] UnhandledPromiseRejectionWarning: Error: send EINVAL 224.0.1.187:5683
**Describe the bug**
Shelly's 2.5 don't get detected anymore since I updated my Homebridge Container to the latest version and the homebridge-shelly plugin to v0.18.1.
Shelly's do have Firmware Version 1.11.0 (untouched since Homebridge updates). Controll of Shelly's with Shelly App is working as normal.

Administration page of plugin does list all the Shellys but they are having the state "unknown" respectively "offline".

**Log output**
```
[8/2/2021, 8:04:08 PM] [Shelly] Admin server is running on port 8181
(node:8282) UnhandledPromiseRejectionWarning: Error: send EINVAL 224.0.1.187:5683
    at doSend (dgram.js:714:16)
    at defaultTriggerAsyncIdScope (internal/async_hooks.js:435:18)
    at afterDns (dgram.js:660:5)
    at processTicksAndRejections (internal/process/task_queues.js:83:21)
(Use `node --trace-warnings ...` to show where the warning was created)
(node:8282) 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: 1)
(node:8282) [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.
```

**Environment:**
 - Shelly device(s): Shelly 2.5
 - Plugin version:  0.18.1
 - Device running homebridge: Docker
