Javascrit error (null reference) is generated when constraint="no empty" is used on a Combobox in a popup / modal window

Fiddle example, recreation of error
http://zkfiddle.org/sample/1hu7pda/8-Combobox-with-constraint-causes-javascript-error-when-placed

1. Press the popup button
2. Select the combobox input field
3. Deselect combobox and close validation waring
4. Close the popup window (generates javascript error)
5. Press the popup button
6. Drop down is no longer working

Error in zk.wpd line 25517, node is null