Reproduction steps:
1) Open project attached in the edit and proceed to scene "MoviePlayer".
2) Notices in the inspector that slider reference under MoviePlayer object is null.
3) Go into playmode.

Note: Removing most of the assets usually makes it not reproducible.

Workaround : Setting slider reference in the inspector resolves this issue.

Reproduced with: 5.4.0b8 , 5.3.3p2 , 5.2.4f1 , 5.1.4f1