I've testing with 4.0, and get the same error. Steps to reproduce:
(1) Open a document and add an item from My Library
(2) Delete the item from My Library, and empty trash
(3) Return to document, open the item for editing and open the Classic View.
(4) The "OK" button will be greyed out, no item shows in the selected list (the right-hand box), and the error will show in Web Developer -> Error Console
The item can be manipulated (to add a page locator etc) with the Quick View dialog. But the Classic View provides the only way of reordering entries currently, so citations with missing items cannot be edited in that respect.
I see now that the error thrown on the string ID is only part of the problem: getFirstCreator() is needed to generate the Classic View listings, and even if fixed to get around the string ID thing, it still returns nothing, so an entry for the item cannot be generated. Not sure how one would go about fixing this one. If reorder-by-drag were implemented/fixed in the Quick View dialog, that would make the breakage in Classic View less of an issue though.
Frank