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That line might be one of your problems, but your painting is definitely also incorrect. That could possibly lead to a bunch of other problems (or cover up problems that are already there), so it's...

Well here's your first problem- you should never use getGraphics() on a component. Extend the JPanel and override paintComponent() instead.

Recommended reading: Painting in AWT and Swing

How does the game "not continue properly"? Do you get an Exception? Does it freeze up? Something else? Have you stepped through the program with a debugger?

PS- Mentioning your deadlines will make...