Null Safety issue

[uc-ach]

This plugin is not null safe, can you update it?

[vin-fandemand]

Yes can you please update it

[xrr2016]

Yes can you please update it

Actually i don‘t know how to do 🤣

[vin-fandemand]

https://github.com/xrr2016/flutter-tree/pull/5

Raised a PR for null safety.

[xrr2016]

It's null safety now!