Upgrade to Null Safety

[AnneBullert]

Hi :)

Thank you for providing this package! You suceeded, where others did not: I had a lot of trouble writing data files (json) to the download dir on some Android versions. With your package it's working like a charm!

I've just opened a Pull request, that contains the migration to null safety.

I would be super happy, if you would publish a new version. This is the last dependency, I need to update, in order to migrate my project to sound null safety ;)

Thank you!

Kind regards,
Anne

[hayelom123]

Error: Cannot run with sound null safety, because the following dependencies
don't support null safety:

• package:document_file_save