Update to null safety

[kitwtnb]

I fixed it for null safety.
#49

[ChemoCosmo]

Thanks a lot. I fear that the developer here has gone dark.
Does your fork also contain the bufixes (#45 and #43) from the other fork #46?.

[kitwtnb]

Yes, I was not aware of that issue, but I fixed it.

[kitwtnb]

You can check it by writing it in pubspec.yaml.

calendarro:
  git:
    url: https://github.com/kitwtnb/calendarro.git
    ref: master