Fixing Non-nullable instance field Errors in Flutter with Null Safety

Learn how to resolve the `Non-nullable instance field '_selectedSize' must be initialized` error in Flutter by utilizing Dart's null safety features, ensuring smoother app performance.
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Fixing Non-nullable instance field Errors in Flutter with Null Safety

When developing a Flutter application, especially one using Dart's null safety features, you may encounter some common issues that can halt your progress. A frequent error that developers face is the Non-nullable instance field '_selectedSize' must be initialized message. This error typically arises due to a variable being declared as non-nullable without being properly initialized. In this guide, we'll take a closer look at this problem and provide a clear solution to overcome it.

Understanding the Problem

In the example you provided, there is a Product class that is designed to manage items for a store app. The problem arises when a non-nullable instance field _selectedSize is defined without being initialized. Here's a summary of what's happening:

When you attempt to access the selectedSize property in your Product class, it defaults to null, causing Dart to throw an error because _selectedSize is declared as non-nullable.

The core of the issue is that your app expects _selectedSize to contain a value (an instance of ItemSize), but the first call to selectedSize tries to access a null value.

The Solution

To resolve this issue, we need to ensure that _selectedSize is properly initialized or marked as nullable. Here’s how to do that step-by-step:

Step 1: Modify the ItemSize Class

First, adjust the ItemSize class to allow for nullable properties. This change will prevent the Dart null safety checks from failing.

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Update the Product Class

Next, modify the Product class to handle the potential null state of _selectedSize. Here’s one way to implement this:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

By implementing these changes, you ensure that your app handles null values more gracefully and avoids the pitfalls of Dart's null safety enforcement. The addition of default constructors and nullable properties will enable your app to function correctly while maintaining the benefits of type safety.

As you continue working with Dart and Flutter, always remember to think critically about how you are declaring and using your variables, especially as you navigate through null safety. Happy coding!