I am not sure what "being used" means, as other attributes that contained a value were allowed to be deleted. I have a attached an image of the Attribute Template dialog box with the error message. If I can figure this out and resolve it then it will fix the issue. Some more context of the errors is below:
If I attempt to change any of the listed attributes (File>Properties>Attributes tab) in the traditional editor, I get an "Internal error: memory access violation" message. If I edit the attribute using the bulk editor, I can set the attribute correctly without the error message, but when I attempt to save the part, I receive an "Attempted to save bad pointer" error. If I then delete the attribute and attempt to save again, I get "The attribute corresponding to an attribute handle cannot be located" error.