null safety not supportable

[reshamRudra]

unable to use on flutter
environment:
sdk: ">=2.12.0 <3.0.0"

[nerdsarena]

same issue with me

[Gazzolla]

Idem

[ihancock]

You can take from my repository for NS support see PR #163

[amitsingh6391]

@ihancock there are some issue related multidex in your repo