How to reproduce:
1. Open the attached project
2. Open videotest scene
3. Select Video Player in hierarchy
4.1 Target Texture has texture assigned, however, click on circle in the right to pick another one
Result: window where one can pick texture doesn't appear and NullReferenceException is thrown
4.2 Click Browse for video URL in the inspector then click Cancel or select video and Open
Result: "NullReferenceException: Object reference not set to an instance of an object" in the console
4.3 Change Audio Output mode to Audio Source, set Controlled Tracks to 1 and try to pick Audio Source
Result: window where one can pick audio source doesn't appear and NullReferenceException is thrown

Reproducible: 2017.3.0b1
Not reproducible: 2017.2.0f1, 2017.3.0a7

Workaround: Drag and drop what you need to that field