This definitely sounds like a bug. I see that in version 3 you cannot easily add an export-to-excel button to a reference set selector; it is not in the context menu. However, you can copy and paste one into its control bar and then you get no error.
This is definitely something we need to look at. Please file a support ticket and we will fix it.
I'm afraid that there is no way around this error for now, except for temporarily deleting the button.