This is occurring in the latest nightly Windows build.

When modifying the order of fields, the 'move up/down' buttons are disabled when at the top/bottom of the list. But if you click on another row, then hold down the mouse button and drag to one of the end rows, the movement buttons are not properly disabled. So you can click to move the top row up and the bottom row down, both of which crash the application.

The inverse also works, so if you click the bottom row, then drag up one row, the "move down" button will be disabled even though you should be able to move the field. So basically it seems like the listener for disabling those buttons triggers on the mouse click, not the actual final selection after dragging around.