Please help with 'Argument Not Optional' error.

I've attached an example workbook that has a custom ribbon with one toggle button on it. The toggle simply 'shows' and 'hides' the formula bar when pressed. The callback in this example works perfect, but when I use the exact same toggle button and callback in my more extensive workbook, I get an 'Argument Not Optional' error. Can anyone explain why this works perfect in the example, but presents an error when used in another workbook. I've read where there is a way to make this optional, if so, how do I go about doing that? Thanks.