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A guide to resolving the `Failed assertion: boolean expression must not be null` error in Flutter when using FutureBuilder to fetch data from Firestore.
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This video is based on the question https://stackoverflow.com/q/62874312/ asked by the user 'Abir Ahsan' ( https://stackoverflow.com/u/12902996/ ) and on the answer https://stackoverflow.com/a/62875106/ provided by the user 'Sisir' ( https://stackoverflow.com/u/3197387/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Failed Assertion: Boolean Expression Must Not Be Null Error in Flutter

When working with Flutter, you might encounter various errors that can disrupt your app's functionality. One such error is the infamous Failed assertion: boolean expression must not be null. If you find yourself grappling with this issue, especially in the context of using a FutureBuilder to fetch data from a Firestore database, you're in the right place.

In this guide, we will walk through the problem and provide you with a clear solution to debug the situation effectively.

The Problem

You may encounter this error while trying to load data asynchronously from Firestore. For instance, consider the following code, which attempts to fetch data from the Firestore and display it using a FutureBuilder:

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

What Went Wrong?

The error arises due to a mistake in checking whether the data has loaded correctly. In the original implementation, the check is performed using:

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

Here, snapshot.data can be null when the data is still being fetched or if an error occurred. This leads to a failed assertion, since !null cannot be evaluated to a boolean.

The Solution

To resolve this issue, you need to change your conditional check to utilize snapshot.hasData, which correctly verifies whether the data is available or not.

Updated Code

Here’s how you can modify the FutureBuilder implementation to avoid this error:

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

Explanation of the Changes

Checking for Data Availability: The updated code now correctly uses snapshot.hasData to proceed when the data is available, thus avoiding the potential for null.

Error Handling: The updated structure also includes a check for snapshot.hasError to handle any errors that occur during the data fetching process, providing a more robust and user-friendly experience.

Loading State: When the data is still loading, a CircularProgressIndicator is shown, providing visual feedback to the user.

Conclusion

Encountering the Failed assertion: boolean expression must not be null error can be frustrating, especially for those new to Flutter. However, with a little understanding of how to properly check for data availability in a FutureBuilder, you can navigate through it effectively. By implementing the solution discussed in this post, you'll be able to fetch and display data from Firestore without running into this assertion error.

If you have questions or further issues, feel free to reach out or share your experiences!