To reproduce:
1. Open attached project;
2. Open "Ray Home" scene;
3. Delete or change something in the scene;
4. Try to save the scene.

Expected result: Editor does not crash on save.
Actual result: Editor crashes on save.

Note: If you delete "ProBuilder Mesh" in the Hierarchy, Editor does not crash.

Reproduced on versions: 5.5.4p1, 5.6.1f1, 5.6.2p2, 2017.1.0f3, 2017.2.0b2.

Note: On versions 5.5.4p1, 2017.1.0f3 and 2017.2.0b2 Fatal Error was thrown: "Could not allocate memory: System out of memory!"