name 'MainWindow' is not defined

This error message typically occurs when you try to reference a variable or object that has not been defined or imported in your code.

In this case, it seems that you are trying to reference a variable or object named 'MainWindow', but it has not been defined anywhere in your code.

To fix this error, you need to define or import the 'MainWindow' variable or object in your code. This will depend on the programming language and framework you are using, so you should consult the documentation or seek help from a tutor or online community to find the appropriate solution.