Problem with adding an existing item to the selected items

[wzol]

I can't seem to figure out how to add an existing item to the already selected items. Example is in this fiddle: https://jsfiddle.net/ba2rgj4z/

I get Uncaught TypeError: Cannot read property 'id' of null

Expected result: both "Test 9" and "Test 6" selected.

Could you help me?