How to close a JFrame based window with a JButton click event?

The problem is that your createGUI method is not static. So I imagine you are creating first a UpgradePopupWindow, calling createGUI on that, which in turn creates a enw UpgradePopupWindow.

Your createGUI() method is a little confused, it creates ANOTHER instance of this class and you end up with one instance with a frame and one instance without. Minimal change to get it working is to change your createGUI method.

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