Steps to reproduce:

• Use Citoid to insert a ref. Insert it and click on something else.
• Decide you don't want that ref. Click 'Undo'.

Results:

• An empty "Basic" citation is created.
• An empty <ref/> tag (https://en.wikipedia.org/w/index.php?diff=656573503&oldid=655723729)

Expected behavior:

If I undo the insertion of the ref, the whole thing should go away.