What does the error message "The method 'call' was called on null" mean in the context of a Flutter app?

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This error message usually means that a method is being called on a variable that has not been initialized or is null. In the context of a Flutter app, it could mean that you are trying to call a function on a variable that has not been assigned a value or has been set to null. To fix the error, you should check to make sure the variable is not null before calling its methods.