Repro steps:
1) Open the project and select any procedural material, that has Standard shader (Road_01 f.e.)
2) Change the shader to Mobile -> Bumped Diffuse
3) Undo the action with Ctrl/CMD + Z
4) Click on the same material
5) Notice the crash

========== OUTPUTING STACK TRACE ==================

0x00000001408423BB (Unity) SubstanceImporter::BindTexturesToShader
0x0000000140845D1E (Unity) SubstanceImporter::OnShaderModified
0x000000014142B8E2 (Unity) SubstanceImporter_CUSTOM_OnShaderModified

Reproduced in: 5.4.0b11; 5.3.4f1; 5.2.4f1