FIX: "Use an existing document" should display linkable documents (fixes

 #242).

Regression caused by 0169054.

frontend/src/components/DocumentList.jsx

@@ -17,7 +17,7 @@ function DocumentList({ relatedTo, verb, setSelectedDocument, setShowDocumentLis
   useEffect(() => {
     backend.refreshDocuments(
       x => setUserDocuments(
-        x.filter(y => y._id !== relatedTo)
+        x.filter(y => y && y._id !== relatedTo)
       )
     );
   }, [user, backend, relatedTo]);