If the mouse is inactive on any moment in time (which actually is quite often), no MouseInfo object is created and your line 3 will set the mouse variable to null. You cannot call any methods on a null value like at line 5. So, line 5 needs to employ another condition: There is another issue with your code. Line 6 calls stop on a GreenfootSound instance that was never started. Each time you use the "new" keyword, you are creating a new -- distinct -- instance of an object of the type specified. You can never use that keyword to make a reference to an object that already exists.

Also, just on coding in general, you would not want to do anything unless you are sure you will need to. As an example: this creates a GameOver object prematurely. This code will probably be invoked on every act cycle and would create a new world every single time. However, only the one created once the score becomes 100 will actually be used. Better is this: where it is created only after the score reaches 100.

You need to first reference the object that holds the sound object in a field. It might be something like: where bgMusic is the name of the GreenfootSound field in your MyWorld class that your MyWorld object will set its music to.