You should give us some code, and some information about what you are doing. It's hard to say anything about just the error without any context. If the code is too much to post, then consider posting a part of it, or at least explain what you are trying to do.
My code about 400 lines and I don't know where the error is, So I will let you know what all I used in my code. I have used sliders, buttons like things to select a file and play it, pause, volume slider,stop are the part of my code and my volume slider and play slider are not working and I am getting this error. Is this error the reason they are not working?