diff a/Source/RunActivity/Viewer3D/Shapes.cs b/Source/RunActivity/Viewer3D/Shapes.cs	(rejected hunks)
@@ -1407,7 +1408,7 @@ public override void Draw(GraphicsDevice graphicsDevice)
         [CallOnThread("Loader")]
         public virtual void Mark()
         {
-            Material.Mark();
+            Material?.Mark();
         }
     }