migrate to null-safety

Hi @redsolver ,

Hope you are all well !

I wanted to try your great project but I was wondering if you can migrate to null-safety ?

Cheers,
Luc

[redsolver]

I don't know if all dependencies support null-safety yet, but I will take a look soon

[mcepl]

I tried to do it in #99, but it is not finished and needs somebody who actually knows Flutter and Dart (which is not me).