Flutter_facebook_login: Support for Null Safety

[Sameerkash]

Null-safety has reached stable on flutter as of 3 march.

Some of the packages I'm trying to migrate depend on this.

I'm happy to make a PR as I need it soon

[darwin-morocho]

this plugin is obsolete, it doesn't have any update since september 2019. Also you need to update the Facebook sdk on ios Android

[mimarko]

this plugin is obsolete, it doesn't have any update since september 2019. You Also need update the Facebook sdk on ios Android

Any other widget for facebook login ?

[darwin-morocho]

this plugin is obsolete, it doesn't have any update since september 2019. You Also need update the Facebook sdk on ios Android

Any other widget for facebook login ?

https://pub.dev/packages/flutter_facebook_auth/versions/3.0.0-nullsafety.1