Looks like we can't reproduce the issue or there's a problem in the test case provided.
Make a Ext.tree.Panel link in Panel or Viewport causes nothing in Design View
Designer version tested:
2.0.0 Build 266
Try making a tree.Panel link in any Panel/Viewport causes nothing to display in Design View. (no problem with grid.Panel). Copy / Move tree.Panel in Panel/Viewport are OK.
Not sure whether it's because of this Build or it's my error.
It was a bug that has since been fixed. Restart the app while online and get the auto-update.