To reproduce:
Open attached project

An error will pop out:
Unloading broken assembly Packages/com.unity.incrementalcompiler/Editor/Plugins/Unity.PureCSharpTests.dll, this assembly can cause crashes in the runtime

Reproduced in package version 0.0.42p29, p21
Not reproduced in p19

--
caused by change to one of Unity's internal API which the IC relies on.

Fixed in 0.0.42-preview.30, however this is breaks support for 2018.3 betas below 11.
For compatibility with beta <= b10 , use 0.0.42-preview.29

so compatible setup is:
Unity 2018.3.b11+ and Incremental Compiler 0.0.42-preview.30
or
Unity 2018.3.b10 and lower and Incremental Compiler 0.0.42-preview.29