Скачать или смотреть Resolving the `Null' is not a subtype of type 'String' Error in Flutter JSON Serialization

Discover how to handle the common Flutter error when dealing with JSON data serialization. Learn about optional parameters and ensuring data availability from APIs.
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This video is based on the question https://stackoverflow.com/q/68361351/ asked by the user 'Nitai Dan' ( https://stackoverflow.com/u/15353570/ ) and on the answer https://stackoverflow.com/a/68361548/ provided by the user 'Stefan Galler' ( https://stackoverflow.com/u/3407244/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving the `Null' is not a subtype of type 'String' Error in Flutter JSON Serialization

If you're developing a Flutter application that interacts with a REST API and processes JSON data, you might encounter the frustrating error: type 'Null' is not a subtype of type 'String'. This issue arises when your app attempts to convert data from JSON into custom Dart objects and finds that some expected data isn’t present. In this guide, we’ll explore this problem in detail and walk through how to effectively solve it.

Understanding the Problem

This error indicates that your application is trying to assign a null value to a variable that expects a String. In the context of your Flutter app, this typically happens while parsing JSON data retrieved from an API. Specifically, in your case, when attempting to create instances of the CallType class, which holds several required fields.

Example Scenario

Suppose the JSON response from the API you're working with should include fields like:

type

contact (which contains first_name and last_name)

created_at

uuid

However, if any of these fields are missing in the actual response from the API, your application will throw the aforementioned error since all properties in the CallType class are marked as required.

Analyzing the Code

Let's take a closer look at the CallType class:

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

The Catch

Notice that if any of the fields (type, first_name, last_name, or created) do not exist in the JSON response, then null will be assigned to those respective fields. Since these are marked as required, the Dart analyzer will raise an error.

Solution: Make Fields Optional

To resolve this issue, you can modify your CallType class by making the fields optional. Here’s how you can do this:

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

Key Changes

Optional Parameters: Remove the required keyword and alter the type to String?. This allows them to accept null.

Default Values: When assigning values in the fromJson factory constructor, use the null-aware operator ?? to set a default value (like an empty string) when the field is absent.

Conclusion

By updating the CallType class to handle nullable values, you will prevent your app from crashing due to missing data from the API. This not only fixes the immediate error but also makes your code more resilient to variations in API responses.

As you continue to build your Flutter applications, always validate your API responses and be prepared to handle unexpected null values gracefully. Happy coding!