phase3: nullfix.py batch script — bang/late/param-nullable

Automated null-safety fixes driven by dart analyze (no corruption):
- unchecked_use_of_nullable_value: insert '!' on receiver (property/method/[]/op)
- not_initialized field/var: mark 'late'
- missing_default_value_for_parameter: nullable param
Errors: 2374(peak) -> 907

lib/widgets/desktop/create_online_store_1.dart

@@ -35,12 +35,12 @@ class CreateOnlineStore1State extends State<CreateOnlineStore1> {
   bool canSubmit = false;
 
   List<dynamic> stores = [];
-  Map<String, dynamic> service;
+  late Map<String, dynamic> service;
   dynamic selectedStore;
 
-  Group group;
+  late Group group;
 
-  String selectedDomain;
+  late String selectedDomain;
   List<dynamic> domainResult = [];
 
   @override
@@ -284,7 +284,7 @@ class CreateOnlineStore1State extends State<CreateOnlineStore1> {
                     ),
                     autofocus: true,
                     validator: (String? value) {
-                      if (value.trim().isEmpty) {
+                      if (value!.trim().isEmpty) {
                         return S.of(context).domains_separated_comma;
                       }
                       return null;