MissingReferenceException: The variable sampleMaterials of UIAssets doesn't exist anymore.

When updating my project to Shapes 3.0.0 I get this exception on Play (as well as project load). It doesn't seem to affect anything from what I can tell however.

What I did was:

- removed my previous 'Shapes' version folder (2.3.1 iirc) (causing a lot of compile errors) 

- imported everything in Shapes 3.0.0 (except for samples)

Full call stack:

MissingReferenceException: The variable sampleMaterials of UIAssets doesn't exist anymore.
You probably need to reassign the sampleMaterials variable of the 'UIAssets' script in the inspector.
Shapes.ShapesImportState.MakeSureSampleMaterialsAreValid () (at Assets/Shapes/Scripts/Editor/Utils/ShapesImportState.cs:80)
Shapes.ShapesImportState.CheckRenderPipeline () (at Assets/Shapes/Scripts/Editor/Utils/ShapesImportState.cs:63)
UnityEditor.EditorApplication.Internal_CallDelayFunctions () (at <007193b7fa9c4ad1be5b26df6a654213>:0)

EDIT:

This doesn't happen when downloading Shapes from a fresh project