Sometimes ItemsControlExtensions throws an ArgumentOutOfRangeException

Hi, I'm working on a project that uses this library. Occasionally we run into an `ArgumentOutOfRangeException` when dragging items. The exception is thrown because of a call to `VisualTreeHelper.GetChild(panel, 0)` when `panel` has no children.

I fixed this by just checking there is at least one child before making the call to `GetChild`, but to be honest I don't really understand the code, so I'm unsure if we are simply using the drag and drop library incorrectly, or if this exception is a bug.