Fix analyzer warning when build an app with this package using Flutter +3.0

[nilsreichardt]

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Removes the warning that you get when building the app:

/opt/hostedtoolcache/flutter/3.13.0-stable/x64/.pub-cache/hosted/pub.dev/number_slide_animation-0.2.1/lib/src/number_slide_animation_widget.dart:53:20:
Warning: Operand of null-aware operation '!' has type 'WidgetsBinding' which excludes null.
 - 'WidgetsBinding' is from 'package:flutter/src/widgets/binding.dart' ('/opt/hostedtoolcache/flutter/3.13.0-stable/x64/packages/flutter/lib/src/widgets/binding.dart').
    WidgetsBinding.instance!.addPostFrameCallback((_) ***
                   ^
/opt/hostedtoolcache/flutter/3.13.0-stable/x64/.pub-cache/hosted/pub.dev/number_slide_animation-0.2.1/lib/src/number_col.dart:39:20:
Warning: Operand of null-aware operation '!' has type 'WidgetsBinding' which excludes null.
 - 'WidgetsBinding' is from 'package:flutter/src/widgets/binding.dart' ('/opt/hostedtoolcache/flutter/3.13.0-stable/x64/packages/flutter/lib/src/widgets/binding.dart').
    WidgetsBinding.instance!.addPostFrameCallback((_) ***