JS error: Drag last issue out of active sprint

Steps to reproduce

1. Start a sprint with one issue in it
2. On Plan Mode, drag the issue in Active Sprint back to the Backlog
3. After confirming, try to drag the issue back into the Active Sprint (which is now empty)

JS error box appears.

Work-around

• After dragging the last issue out of an Active Sprint, reload the page. The sprint will no longer appear.