Reproduction steps:
1. Open attached project "2017.3 Example Project - 360 Video.zip";

Actual results: Editor crashes while loading the project.
Expected results: the project will be opened without crashes/errors.

Note: the project contains 360 degrees video, which causes the crash.

Reproduced on: 2017.3.0a1, 2017.3.0b1.
Not reproduced: 2017.2.0f2, 2017.2.0a1.
Regression firstly introduced: 2017.3.0a1.

The issue happens when the project being upgraded has a VideoClip that had failed to import in the original version of Unity.
The workaround is to either make a proper import in the original version (remove the options that caused the import to fail or remove the clip entirely) and then go to the newer version to upgrade.