fix: remove extension for FormBuilderState; add functions directly to class

Author: danvick

lib/flutter_form_builder.dart

@@ -19,5 +19,4 @@ export './src/form_builder_field.dart';
 export './src/form_builder_field_option.dart';
 export './src/form_builder_validators.dart';
 export './src/widgets/grouped_checkbox.dart';
-export './src/widgets/grouped_radio.dart';
-export './src/utils/extension.dart';
+export './src/widgets/grouped_radio.dart';

lib/src/form_builder.dart

@@ -148,6 +148,12 @@ class FormBuilderState extends State<FormBuilder> {
     _formKey.currentState!.save();
   }
 
+  void invalidateField({required String name, String? errorText}) =>
+      fields[name]?.invalidate(errorText ?? '');
+
+  void invalidateFirstField({required String errorText}) =>
+      fields.values.first.invalidate(errorText);
+
   bool validate() {
     final validation = _formKey.currentState!.validate();
     if (!validation) {