check null safety

lib/domain/entities/carton_size.dart

@@ -1,10 +1,11 @@
 class CartonSize {
-  String id;
-  String name;
+  String? id;
+  String? name;
   double length;
   double width;
   double height;
-  CartonSize({this.id, this.name, this.length, this.width, this.height});
+  CartonSize(
+      {this.id, this.name, this.length = 0, this.width = 0, this.height = 0});
 
   Map<String, dynamic> toMap() {
     return {
@@ -27,19 +28,10 @@ class CartonSize {
   }
 
   @override
-  bool operator ==(other) {
-    if (identical(this, other)) {
-      return true;
-    }
-    return other.id == this.id;
-  }
+  bool operator ==(Object other) => other is CartonSize && other.id == id;
 
   @override
-  int get hashCode {
-    int result = 17;
-    result = 37 * result + id.hashCode;
-    return result;
-  }
+  int get hashCode => id.hashCode;
 
   bool isChangedForEdit(CartonSize cartonSize) {
     return cartonSize.name != this.name ||