Steps to reproduce:
1. Start a new project
2. Go to project view and add a Shader
3. Add a Shader Variant Collection and assign the created shader
4. Delete the shader and select back to Shader Variant Collection
5. Observe the NullReferenceException output in the console and broken Shader Variant Collection display

Expected result: Shader Variant should display "missing shader" instead of NullReferenceException

Fixed in: 2018.3.0a11, 2018.2.7f1, 2018.2.0a3
Reproduced in: 2018.2.0a2, 2018.1.9f1, 2017.4.11f1, 2017.2.3p1