Null safety migration

[sunderee]

This pull request addresses migration to Dart's new safety feature and it brings many improvements related to lint rules which haven't been followed thus far (for instance, preferring var).

[pawelpetruch]

@alfonsocejudo can you merge this and publish a new pub.dev?

[alfonsocejudo]

Wow nicely done

[PaoloP98]

I have an error using the null safety version

E/flutter ( 687): [ERROR:flutter/lib/ui/ui_dart_state.cc(186)] Unhandled Exception: type '(BaseCluster, double, double) => MapMarker' is not a subtype of type '((BaseCluster?, double?, double?) => MapMarker)?'

I initialised the cluster in this way:

      minZoom: minZoom,
      maxZoom: maxZoom,
      radius: 150,
      extent: 4096,
      nodeSize: 64,
      points: markers,
      createCluster: (
        BaseCluster cluster,
        double lng,
        double lat,
      ) =>
          MapMarker(
        id: cluster.id.toString(),
        latitude: lat,
        longitude: lng,
        isCluster: cluster.isCluster,
        clusterId: cluster.id,
        pointsSize: cluster.pointsSize,
        childMarkerId: cluster.childMarkerId,
      ),
    );

Do you know what the problem is?