add submit widget for mix carton

lib/pages/carton/model/carton_selection_model.dart

@@ -15,6 +15,8 @@ class CartonSelectionModel extends BaseModel {
   bool reachEnd = false;
   List<Carton> cartons = [];
 
+  bool isLoading= false;
+
   // for default carton
   DocumentSnapshot? _lastDocument;
   bool ended = false;
@@ -96,6 +98,7 @@ class CartonSelectionModel extends BaseModel {
     int rowPerPage = 20;
 
     try {
+      isLoading = true;
       String path = "/$cartons_collection";
       Query query = FirebaseFirestore.instance
           .collection(path)
@@ -131,6 +134,8 @@ class CartonSelectionModel extends BaseModel {
       notifyListeners();
     } catch (e) {
       log.warning("error:$e");
+    }finally{
+      isLoading = false;
     }
   }