type 'Null' is not a subtype of type 'String'

Why This Happens

In Dart with null safety, non-nullable types cannot hold null. This error occurs when a null value flows into a non-nullable String parameter, often from JSON parsing or API responses. You should declare the type as nullable (String?) or provide a default value.

The Problem

final Map<String, dynamic> json = {'name': null};
final String name = json['name']; // null assigned to String

The Fix

final Map<String, dynamic> json = {'name': null};
final String name = json['name'] ?? 'Unknown';

Step-by-Step Fix

1. 1

   Identify the error

   Look at the error message: type 'Null' is not a subtype of type 'String'. This indicates a null value where a non-nullable type was expected.

2. 2

   Find the cause

   Check JSON parsing, API responses, or map lookups where values might be null but the receiving variable is non-nullable.

3. 3

   Apply the fix

   Use nullable types (String?), provide default values with ??, or validate data before assignment.