Support null safety

The library 'package:flutter_sidekick/flutter_sidekick.dart' is legacy, and should not be imported into a null safe library.
Try migrating the imported library.

```
Error: This project cannot run with sound null safety, because one or more project dependencies do not
support null safety:

-  package:flutter_sidekick
```

```
dependencies:
  flutter_sidekick: ^0.1.3
```