This is only a minor bug. If you scan your project for available strings with the String Table Manager and then click on "Manage strings", right-click on one of the found strings, assign string ID and then click into an empty space in the upcoming selection list instead of selecting an existing item, there will be an error. From there you can click for Details, Continue or Exit. If you ever experience this error, click on Continue, there should be no harm done and you can resume as usual.
This error does not appear if you already made a selection correctly and then try to click into the empty space. You have to made no selection at all in the "fresh" window and then click into the empty space to produce this error.