How to enable user input?

Question

I am building my own field within a 'FormBuilder' using 'FormBuilderField'. Input is disabled after this. I tried Flutter Form Builder document.

FormBuilderField(
          name: 'userName',
          onChanged: (value) => {},
          validator: FormBuilderValidators.compose([
            FormBuilderValidators.required(),
          ]),
          builder: (FormFieldState<dynamic> field) {
            return InputDecorator(
                decoration: InputDecoration(
                    label: const Text('User name'),
                    errorText: field.errorText));
          },
        ),