bug(serial): [ERROR:trollius]: Future/Task exception was never retrieved... WindowsError: [Error 6] The handle is invalid

As of MicroDrop 2.19, when the `mr-box-plugin` (and probably the `dropbot-plugin`) loads the following error pops up:

```py
[ERROR:trollius]: Future/Task exception was never retrieved

WindowsError: [Error 6] The handle is invalid
```