NullReferenceException from ShapeGroup on entering play mode in 2019.4.10f1 LTS with URP

I'm getting the following NullReferenceException whenever I enter play mode with a shape that is tinted by a ShapeGroup:

NullReferenceException: Object reference not set to an instance of an object

Shapes.ShapeRenderer.UpdateMaterial () (at Assets/Shapes/Scripts/Runtime/Components/ShapeRenderer.cs:286)

Shapes.ShapeRenderer.UpdateAllMaterialProperties () (at Assets/Shapes/Scripts/Runtime/Components/ShapeRenderer.cs:363)

Shapes.ShapeGroup.UpdateChildShapes () (at Assets/Shapes/Scripts/Runtime/Components/ShapeGroup.cs:46)

Shapes.ShapeGroup.OnValidate () (at Assets/Shapes/Scripts/Runtime/Components/ShapeGroup.cs:40)

After the exception everything seems to work just fine.

I'm using Unity 2019.4.10f1 LTS and Shapes 2.3.1, this happens in a fresh project.

Also worth noting is that I'm using URP, although I haven't tried this with built-in or HDRP.

Reproduction steps:

-Create new project with Unity 2019.4.10f1 (Universal Render Pipeline Template)

-Import the Shapes package (version 2.3.1), accepting the dialog during import to reload shaders for URP.

-Add a Line object to the scene.

-Add a ShapeGroup onto a new empty game object and parent the Line to it.

-Enter play mode, this triggers the exception.

-The exception is also triggered again upon exiting play mode or reloading the scene.