Setting DD.node to null instead of deleting

In the footsteps of my previous commit. This one replaces the deletion
of DD.node with setting it to null.

src/DragAndDrop.ts

@@ -93,7 +93,7 @@ export const DD = {
         }
       }
 
-      delete DD.node;
+      DD.node = null;
 
       if (layer || node instanceof getGlobalKonva().Stage) {
         (layer || node).draw();