migrated to null safety

[Faiizii]

please have a look and let me know if any further changes required. I migrated the project to null-safety and run flutter create . command in example directory to create ios, android and web project.

[amariucaitheodor]

Awesome! Thank you for this.
Don't know about the other changes, but it's a shame null safety hasn't been merged into the library yet.

[lsaudon]

I think we should fork and create a new package.
It is no longer active since 2019.