To repro:
1. Create project, add new Cube to scene.
2. Add component Mesh filter.
3. In appeared dialog window press Cancel.

Actual result: Exception falls:
MissingReferenceException: The object of type 'ContainerWindow' has been destroyed but you are still trying to access it.
Your script should either check if it is null or you should not destroy the object.

Expected result: No exception happens.

Reproduces on:
2018.2.0a2 - 2018.2.0a5

Does not reproduce on:
Version 2018.1.0b13 (43de91b8ac41)
Thu, 22 Mar 2018 22:23:22 GMT
Branch: 2018.1/staging