Crash when duplicating camera on sequencer with viewport closed

Can be worked around with always keeping a viewport opened.

https://udn.unrealengine.com/s/question/0D5QP000002Hfdh0AC

[Link Removed]

Repro steps (project found at this UDN link)

Close the viewport if it is open

・Double-click LS_Test to open it

・Copy CineCameraActor and paste it onto the sequence

・Crash