The method 'add' can't be unconditionally invoked because the receiver can be 'null'. Using for in loop to get a list of data. Flutter

Question

I'm new to programming and I'm learning flutter. I'm trying to loop through a list of _tasks to get the tasks that has the parentId I need.

• Error: The method 'add' can't be unconditionally invoked because the receiver can be 'null'.
• Question 1: I already make sure the item is not null in the if statement, but the add method still think the item could be null.. How do I prevent this?
• Question 2: If there a better way to get a list of Tasks without using for in loop?

class TaskData extends ChangeNotifier {
  List<Task> _tasks = [
  Task(parentId: 1, name: "Task1"),
  Task(parentId: 1, name: "Task2"),
  Task(parentId: 2, name: "Task3"),
  Task(parentId: 3, name: "Task3"),
  ];

  List<Task>? getTasksByParentId(int parentId) {
    List<Task>? TasksWithSameListId;

    if (_tasks.isNotEmpty) {
      for (var item in _tasks) {
        if (item.parentId == parentId) {
          TasksWithSameListId.add(item);
        }
      }
    }
  }

Expecting result: when calling function getTasksByParentId, it should return a list of Tasks that has the same parentId.

Answer 1

The error occurs because your variable TasksWithSameListId is nullable. Also never initialized. To fix your issue you can initialize it and make it non nullable.

List<Task> getTasksByParentId(int parentId) {
  final List<Task> tasksWithSameId = [];

  if (_tasks.isNotEmpty) {
    for (var item in _tasks) {
      if (item.parentId == parentId) {
        tasksWithSameId.add(item);
      }
    }
  }
  return tasksWithSameId;
}

A better approach is to use where to filter your list.

List<Task> getTasksByParentId(int parentId) {
  return _tasks.where((item) => item.parentId == parentId).toList();
}