Is it possible to create an Excel 2007 add-in that checks the
existence for a particular sheet in an workbook and if that sheet
exists populate drop-downs on a ribbon with values from that
particular sheet?

I am curious b/c I have created a workbook that does what I want
(populates drop-downs if the sheet exists in the active workbook) but
as soon as I save the file as an add-in and try to use it on other
workbooks I keep getting an Error 91 (object variable) when I try to
Invalidate the ribbon and refresh the drop-down values.

Thanks,
-Brian