Unsupported operation on an unmodifiable list

Why This Happens

In Dart, some lists are unmodifiable (e.g., const lists, List.unmodifiable). Attempting to add, remove, or modify elements throws an UnsupportedError. Create a new modifiable copy of the list before making changes.

The Problem

final items = const [1, 2, 3];
items.add(4); // Cannot modify const list!

The Fix

final items = const [1, 2, 3];
final mutableItems = List<int>.from(items);
mutableItems.add(4);

Step-by-Step Fix

1. 1

   Identify the error

   Look at the error message: Unsupported operation: Cannot add to an unmodifiable list. This means the list cannot be modified.

2. 2

   Find the cause

   Check if the list was created with const, List.unmodifiable, or returned from a method that provides an unmodifiable view.

3. 3

   Apply the fix

   Create a mutable copy with List.from() or .toList() before modifying the list.