Reproduction Steps:
1. Import attached project.
2. Open "Test" scene.
3. In Hierarchy, select "Not Transcoded".
4. In Inspector, click the object selection button in the Video Clip parameter.
5. Select "Not Transcoded".
6. Click on the preview window to play the clip.

Expected result: Clip plays in the preview window.
Actual result: Console gives out error (GIF attached):

VideoManager::StartPreview() : Invalid VideoClip
VideoManager::PlayPreview() : Invalid Preview

Note #1: An additional "Transcoded" object is attached to show how the preview should work.

Reproducible with: 5.6.0b11, 2017.1.0a3

Note #2: Not reproducible with previous versions.