The default Iris configuration file pretends that shaders are enabled, when they're not.

[Lolothepro]

Iris/src/main/java/net/irisshaders/iris/config/IrisConfig.java

Line 41 in 1d01bf0

enableShaders = true;

enableShaders cannot be true if shaderPackName is null

[mrjasonn]

I would suggest that the fix to be that if shaderPackName is null, disable enableShaders. If the specified shaderpack does not exist, make the shaderPackName field null and disable enableShaders. The default config will also have to be fixed.