If you are using the null check operator (??) in Flutter and you are getting an error saying that it is being used on a null value, it means that the value being checked is null and the null check operator is trying to access a property or method on it.

To fix this error, you will need to make sure that the value is not null before using the null check operator. You can do this by using an if statement to check if the value is null, and if it is not null, then you can use the null check operator.

Here is an example of how you can do this:

if (myValue != null) {
print(myValue.property ?? 'Default value');
}

In this example, myValue is checked to see if it is null. If it is not null, then the property of myValue is accessed using the null check operator. If myValue.property is null, then the string 'Default value' will be printed. If myValue.property is not null, then its value will be printed.