Null Safety support

[iqfareez]

Reload already in progress, ignoring requestError: Cannot run with sound null safety, because the following dependencies
don't support null safety:

 - package:flutter_timetable_view
2

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

Anyway @yamarkz, seems like the latest master is migrated to the null safety. Would you mind publishing the latest version to the pub.dev? Thanks!