How to Fix the Argument Type 'null' Error in Flutter

If you're working with Flutter, you may encounter an error message that says "the argument type 'null' can't be assigned to the parameter type 'key'." This error occurs when you're trying to pass a null value to a method or constructor that expects a non-null argument. In this tutorial, we'll show you how to fix this error in Flutter.

Check the Widget Tree: The first step in fixing this error is to check the widget tree to see where the null value is being passed. Look for any widget that's missing a required parameter, or any widget that has a null value assigned to a required parameter. Once you've identified the widget causing the error, you can move on to the next step.

Provide a Non-Null Value: The easiest way to fix this error is to provide a non-null value for the parameter that's causing the error. If the parameter is a String, you can provide an empty string ('') instead of null. If the parameter is a Widget, you can provide a default widget as a fallback.

Container(
  key: UniqueKey(),
  child: Text('Hello World'),
)

Use a Key: If the parameter causing the error is the 'key' parameter, you can fix the error by providing a Key. Keys are used to uniquely identify a widget in the widget tree, and they should be unique and consistent for each widget. You can use the 'ValueKey' or 'UniqueKey' class to generate a key.

Container(
  key: ValueKey('myContainer'),
  child: Text('Hello World'),
)

Container(
  key: UniqueKey(),
  child: Text('Hello World'),
)

That's it! By following these steps, you should be able to fix the argument type 'null' error in Flutter.

1 year ago