Hi Beth,
Thanks, I'm able to produce the problem. I will look into it. The issue seems to be that adding features happens inside an updatepanel when you click the arrow button. It really doesn't require clicking the save button at that point, but it seems that something about the state of the page after it has done an ajax postback causes this error when the outer page does post back from the save button.
I'll look into this and try to get a fix into the repository soon. In the mean time you can avoid the error by refreshing the page after adding/removing features.
Best,
Joe