Resolving the ‘type ‘Null’ is not a subtype of type ‘String” Error in Flutter

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With its strong architecture and adaptable UI toolkit, Flutter enables developers to create cross-platform apps quickly and easily. But much like any other programming environment, Flutter occasionally produces errors that could be confusing to developers. One such error is “type ‘Null’ is not a subtype of type ‘String.”

Error: ‘type ‘Null’ is not a subtype of type ‘String’

When you try to use a value that might be null as if it were a String, you usually get the error message “type ‘Null’ is not a subtype of type ‘String’.” When working with variables or data acquired from other sources, like databases or APIs, in Flutter, this problem frequently occurs.

Understanding the Problem

Let’s dissect the error message and understand its underlying cause. Consider the following scenario

late String _dropdownValue;

void initState() {
  _dropdownValue = widget.registrationData['regcouncil'] ?? 'Select Council';

This code snippet initialises _dropdownValue to a String type. Nevertheless, the ‘Select Council’ string is assigned to _dropdownValue using the?? operator if widget.registrationData[‘regcouncil’] is null. The problem arises because widget.registrationData[‘regcouncil’] may be null, even if _dropdownValue is meant to be of type String.


The ‘type ‘Null’ is not a subtype of type ‘String’ problem must be fixed by making sure _dropdownValue is capable of handling null values. This is the code that we can change to fix the problem.

late String? _dropdownValue; // Make _dropdownValue nullable

void initState() {
  _dropdownValue = widget.registrationData['regcouncil'] as String? ?? 'Select Council';

_dropdownValue has been modified to be of type String?, rendering it nullable.
InitState()’s initialisation has been modified to properly handle null values. The value of widget.registrationData[‘regcouncil’] is now allocated to _dropdownValue, provided that it is not null. “Select Council” is used as the default value if it is null.

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