Nonetype is not iterable almost always means you asked for a list and Maya gave you back a 'None' instead of an empty list. You can get around the error in the above code with
selectedLights = cmds.textScrollList ("lgtList", query = True,
selectItem = True) or []
Which will always return a list, although the list will be empty if nothing is selected in your UI.
For more on how to handle list-type returns in Maya, see my answer to this thread