handleFocusChange method
Implementation
void handleFocusChange(int index) {
  FocusNode? focusNode = _focusNodes[index];
  TextEditingController? controller = _textControllers[index];
  if (focusNode == null || controller == null) return;
  if (focusNode.hasFocus) {
    controller.selection = TextSelection.fromPosition(
        TextPosition(offset: controller.text.length));
  }
}