GUI -- Protect against non-existent link element.

Change-Id: I6638de69c9d826c7697468727004c4392bf4925a

@@ -874,8 +874,13 @@
     }
 
     function linkExisting(d) {
+        // this is supposed to be an existing link, but we have observed
+        //  occasions (where links are deleted and added rapidly?) where
+        //  the DOM element has not been defined. So protection against that...
+        if (d.el) {
             restyleLinkElement(d, true);
         }
+    }
 
     function linkEntering(d) {
         var link = d3.select(this);