Fix widgets binding warning

[tainanfochesatto]

This PR fixes a null-safety warning that came with upgrading to flutter 3.

Fixes #125

[leonus96]

@jifalops

[MrCsabaToth]

@jifalops simple fix, please merge and release a new version

[tylergets]

The project hasn't seen activity in 1.5 years, might be a good idea to fork as the author seems unresponsive.

In the meantime, you can update your pubspec to:

  datetime_picker_formfield:
    git:
      url: https://github.com/tainanfochesatto/datetime_picker_formfield.git
      ref: fix-widgets-binding-warning

[mikeesouth]

Ping @jifalops, you're active on github so if you have time to approve this PR and push a new version to pub.dev, that would be really helpful. Thanks!

[MrCsabaToth]

@jifalops ?