Why does calling dispose() on Graphics object cause JPanel to not render any components

The thing is that the Graphics context you are using in paintComponent is created and provided by the caller (the framework), which is also responsible for disposing of it.

You only need to dispose of Graphics when you actually create it yourself (for example by calling Component.getGraphics()). In your case, you’re not creating it, you’re just casting it, so do not call dispose in this case.