implemented way to add todo

lib/models/todo.dart

@@ -1,5 +1,7 @@
 import 'package:isar/isar.dart';
 
+import '../services/dbhelper.dart';
+
 part 'todo.g.dart';
 
 @collection
@@ -10,9 +12,10 @@ class Todo {
   final DateTime createdAt;
 
   Todo({
-    required this.id,
+    int? id,
     required this.title,
     this.description = '',
     DateTime? createdAt,
-  }) : createdAt = createdAt ?? DateTime.now();
+  })  : createdAt = createdAt ?? DateTime.now(),
+        id = id ?? DbHelper.nextTodoId;
 }