Remove warnings about Operand of null-aware operation on WidgetsBinding

[hoffmannkrzysztof]

In flutter3.0 WidgetsBinding and SchedulerBinding are now non-nullable. To be backward compatible with flutter2.x and get rid of warning we need to implement this solution: https://docs.flutter.dev/development/tools/sdk/release-notes/release-notes-3.0.0#if-you-see-warnings-about-bindings

[aaassseee]

@WahdanZ Any time for review? This will fix #309

[vasilich6107]

@hoffmannkrzysztof

This is not the first time when form_bloc has issues with delivery

Today I removed last piece of code and form_bloc as dependancy.

I recommend you pay attaention to reactive_forms

• form itself https://pub.dev/packages/reactive_forms
• a lot of prebuilt widgets ready to use https://github.com/artflutter/reactive_forms_widgets
• code gen for strictly typed forms https://github.com/artflutter/reactive_forms_generator

[rlimberger]

Can you please merge this fix, @GiancarloCode ? Thank you.

[vasilich6107]

@rlimberger "All hope abandon ye who enter here"(Dante) ;-)

[GiancarloCode]

LGTM