Resolving Unhandled Exception: Invalid argument(s) (value): Must not be null in Flutter 2

Описание: A guide to troubleshooting the 'Invalid argument' error when using SharedPreferences in Flutter 2, including code updates and best practices.
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This video is based on the question https://stackoverflow.com/q/67720594/ asked by the user 'M Al' ( https://stackoverflow.com/u/14529935/ ) and on the answer https://stackoverflow.com/a/67721726/ provided by the user 'Reza M' ( https://stackoverflow.com/u/6600475/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Troubleshooting the 'Unhandled Exception: Invalid argument(s) (value): Must not be null' Error in Flutter 2

If you've recently switched to Flutter 2 and found yourself facing an Unhandled Exception: Invalid argument(s) (value): Must not be null error when saving values using SharedPreferences, you're not alone. This issue typically arises due to a change in how null values are handled in the new version of Flutter. Let's dive into the problem and explore how to resolve it.

Understanding the Error

The error message you’re encountering indicates that you're attempting to set a null argument where a valid non-null value is required. Specifically, it occurs during the execution of methods that save data in SharedPreferences if any of the values being saved are not properly initialized.

Example Error Log

The error log might look something like this:

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

This log clearly indicates that the value you're trying to save in SharedPreferences must not be null.

The Problem Explained

In your existing code, you’re trying to save the language preference of the user. The language preference values are determined based on the user’s selection from a radio button interface. When the selection is made, you set the corresponding value and attempt to save it.

Code Snippet Causing the Issue

Here's a relevant part of your code:

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

Despite handling options, if _myPreferences.lang is null at any point, the commit method will invoke a failure.

Solution Steps

To fix this issue, we need to ensure that the lang variable in your SharedPreferences instance is never null during the save operation. Here’s how you can update your commit() method to handle this gracefully.

Updated commit Method

Modify your commit method in the MyPreferences class as follows:

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

Explanation of the Changes

Null Check: The method first checks if lang is not null. If it is valid, it saves the value directly.

Setting Default: If lang turns out to be null, it assigns a default value (in this case, 1 for English) before saving.

Returns Status: The method returns a boolean to indicate whether it saved the intended value or a default instead.

Conclusion

By making the above changes, you should see the Unhandled Exception error vanish when running your Flutter application. Remember to handle user input carefully and always ensure that variables are initialized before being used.

This approach not only resolves current issues but also prevents potential future errors related to null values. As you continue to improve your app using Flutter, keeping an eye on such nuances will improve the stability and user experience of your applications.

Happy coding!