Fix null `DataList` issue + other null-safety concerns

[pierrea]

This PR fixes https://github.com/Dn-a/flutter_tags/issues/79 and addresses all the other null-safety warnings. Version proposed: 1.0.1

[smalburg1]

It worked! Why has this not been merged yet?

[pierrea]

cc @Dn-a 🙏

[tony123S]

so how do we use this if this has not been merged yet ?!

[pedrox-hs]

so how do we use this if this has not been merged yet ?!

@tony123S, sorry for the delay, I think you already did it, but if anyone needs it too:

flutter pub add flutter_tags --git-url=https://github.com/pierrea/flutter_tags --git-ref=null-safety