Null Safety

[josxha]

I have updated the package to unsound null safety because I needed it migrated to test my implementation.
However, I recommend to wait with merging this PR until a version with sound null safety is possible.

The following procedure is still missing for this:

1. flutter_image must be migrated to sound null safety (https://github.com/flutter/flutter_image/pull/22)
2. flutter_map must be migrated to sound null safety (Upgrade to sound null-safety fleaflet/flutter_map#870)
3. the dependency_overrides entry must be removed in the pubspec.yaml file.

I tested my PR with the example app and my own application and I didn't notice any problems.

If you already want to use the null safety version, you can do so as follows:

dependencies:
  ....
  lat_lon_grid_plugin: any

dependency_overrides:
  lat_lon_grid_plugin:
    git:
      url: git://github.com/josxha/lat_lon_grid_plugin.git
      ref: 324bbb630af27829f47d30cad3e2fc4a97c29ef7

[josxha]

Just updated my pull request to use the published version of flutter_map from pub.dev. I will test the package again with my own implementation and the example project.

[josxha]

I have tested the package and everything runs without problems.
@mat8854 The pull request can now be merged and published to pub.dev if you find the time.