Hi while i am trying to run your code i am getting the following error.

../Flutter/flutter_windows_3.3.10-stable/flutter/.pub-cache/hosted/pub.dartlang.org/nb_utils-5.0.8/lib/src/widgets/overlay_custom_widget.dart:99:25: Error: Method 'insert' cannot be called on 'OverlayState?' because it is potentially null.

• 'OverlayState' is from 'package:flutter/src/widgets/overlay.dart' ('../Flutter/flutter_windows_3.3.10-stable/flutter/packages/flutter/lib/src/widgets/overlay.dart').
  Try calling using ?. instead.
  Overlay.of(context).insert(entry);
  ^^^^^^
  ../Flutter/flutter_windows_3.3.10-stable/flutter/.pub-cache/hosted/pub.dartlang.org/nb_utils-5.0.8/lib/src/widgets/app_text_field.dart:65:9: Error: Type 'EditableTextContextMenuBuilder' not found.
  final EditableTextContextMenuBuilder? contextMenuBuilder;
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  ../Flutter/flutter_windows_3.3.10-stable/flutter/.pub-cache/hosted/pub.dartlang.org/nb_utils-5.0.8/lib/src/widgets/app_text_field.dart:65:9: Error: 'EditableTextContextMenuBuilder' isn't a type.
  final EditableTextContextMenuBuilder? contextMenuBuilder;
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  ../Flutter/flutter_windows_3.3.10-stable/flutter/.pub-cache/hosted/pub.dartlang.org/nb_utils-5.0.8/lib/src/widgets/app_text_field.dart:353:7: Error: No named parameter with the name 'contextMenuBuilder'.
  contextMenuBuilder: widget.contextMenuBuilder,
  ^^^^^^^^^^^^^^^^^^
  ../Flutter/flutter_windows_3.3.10-stable/flutter/packages/flutter/lib/src/material/text_form_field.dart:97:3: Context: Found this candidate, but the arguments don't match.
  TextFormField({
  ^^^^^^^^^^^^^

FAILURE: Build failed with an exception.

Thanks,
Suresh.

I would like to modify it's theme? It will look more cool in modern theme.