Null safety, dart 3 & flutter 3.13.x, updated test dependency, fixed data chunk issue

[novas1r1]

Hey, since I need this package for my current app I forked it and created some changes.

• updated to null safety
• updated to dart 3 and flutter 3.13.x
• updated flutter_test dependency
• included bugfix from @dhcracchiolo, see #4

Would be great to see this merged :)