Steps to reproduce:

1) Download attached project 'BugReproShaderVariants.zip' and open in Unity
2) In the Project window, click on 'ShaderVariantsForMany.shadervariants' Shader Variant Collection
3) In the Inspector window, click on 'Add shader' button
4) After shader selection window shows up, double click on 'ManyVariantsShader.shader' shader
5) Click on the plus icon to add a shader variant to Shader Variant Collection
----Unity will freeze

Expected result: Unity should add a shader variant to Shader Variant Collection without freezing
Actual result: Unity freezes while adding a shader variant to Shader Variant Collection

Reproduced with:
5.5.4p2, 5.6.3f1, 2017.1.0p3, 2017.2.0b6, 2017.3.0a2