Fix tooltip issue causing exception when node becomes cluster

lib/network/modules/InteractionHandler.js

@@ -694,7 +694,7 @@ class InteractionHandler {
         // we initially only check stillOnObj because this is much faster.
         if (stillOnObj === true) {
           let overNode = this.selectionHandler.getNodeAt(pointer);
-          stillOnObj = overNode.id === this.popup.popupTargetId;
+          stillOnObj = overNode === undefined ? false : overNode.id === this.popup.popupTargetId;
         }
       }
     }