x_Icon null safety compile warnings

[asdef]

When x_icon ^0.0.5 is included in a project, when that project is compiled (discovered during unit test execution) it throws two warnings in icon.dart:

/home/asdef/.pub-cache/hosted/pub.dartlang.org/x_icon-0.0.5/lib/src/icon.dart:51:24: Warning: Operand of null-aware operation '!' has type 'ThemeData' which excludes null.
 - 'ThemeData' is from 'package:flutter/src/material/theme_data.dart' ('/flutter/flutter/packages/flutter/lib/src/material/theme_data.dart').
          style: Theme.of(context)!
                       ^
/home/asdef/.pub-cache/hosted/pub.dartlang.org/x_icon-0.0.5/lib/src/icon.dart:52:16: Warning: Operand of null-aware operation '!' has type 'TextTheme' which excludes null.
 - 'TextTheme' is from 'package:flutter/src/material/text_theme.dart' ('/flutter/flutter/packages/flutter/lib/src/material/text_theme.dart').
              .textTheme!
               ^

To Reproduce

1. add x_icon to any flutter project
2. compile any code in the project, it doesn't even need to use x_icon
3. receive compile warnings

Expected behavior
no compile warnings.

Additional context
Flutter version: 2.2.3
dart version: 2.13.4

[asdef]

Created pull request: #27