Draggable: stack option can throw error

From a forum post:

The fact that draggable stack option is now a selector apparently introduced a bug. If no elements exist that match the selector, you get an error on drag (jquery-ui.js line 1341 group[0] is undefined). A simple check for an element at 0 would fix this bug. (my use case is that the dragged element gets assigned the class that matches this selector on drop, and subsequent drags of other elements would stay on top of previously dropped elements)