Support null safety

[ytyng]

[✓] Flutter (Channel stable, 2.2.1, on macOS 11.2.3 20D91 darwin-x64, locale ja)

Import statement calls should not be imported into a null safe library.

When building,

Launching lib/main.dart on sdk gphone x86 arm in debug mode...
Running Gradle task 'assembleDebug'...
Error: Cannot run with sound null safety, because the following dependencies
don't support null safety:

 - package:flutter_sunmi_printer

For solutions, see https://dart.dev/go/unsound-null-safety


FAILURE: Build failed with an exception.

Thank you.

[GarrettLi77]

+1

[Jonakss]

+1

[azhansy]

#15

[jorger5]

+1

thanks @azhansy hope your PR gets merged soon!