Nanite material audit reports null material when WorldGridMaterial replaces a deleted material in the editor

Workaround it to reassign a material to the mesh and resave

1. Create a dummy material and assign it to a StaticMesh with Nanite enabled (in the Static Mesh Asset Editor).
2. Delete (select Force Delete) the material. The material will now be replaced with the WorldGridMaterial in the Static Mesh Asset Editor.
3. Nanite material audit now complains that the mesh has a null material even though it looks like it has the WorldGridMaterial assigned. Saving the mesh doesn't resolve this.