Fixing SliverStickyHeaderState cast error due to null-safety migration

[amitkot]

Solves #62

@letsar please review, thanks!

[StorminGorman]

Is development on this halted? I'm tempted to just add a git dep to your PR fork .....

[amitkot]

That what I did for now.
@letsar would it be possible to merge this? I am currently using it in production on Android and iOS (https://bringitapp.co).

[ThexXTURBOXx]

For the time being just use

dependency_overrides:
  flutter_sticky_header:
    git:
      url: https://github.com/amitkot/flutter_sticky_header.git
      ref: master

[oscarshaitan]

this should be merged months ago!!!!

[timcreatedit]

@letsar Pleeeease merge this pull request, it should be really quick and returns this great package to being usable

[oscarshaitan]

I get tired and create my own widgets with the help of Sliver_tools check that
https://pub.dev/packages/sliver_tools

[letsar]

Thank you a lot, I merged it and published a 0.6.1 version on pub