[2D] [Sprite Shape] Error generated when using a default sprite for Corner sprite or Angle Range sprite in Sprite Shape Profile

Steps to Repro:
1. install Sprite Shape package
2. create a new Sprite Shape Profile
3. In either Corners or add an Angle Range sprite- select a default sprite (e.g. UISprite)
4. Observe error messages are generated in the console.

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.U2D.SpriteShapeEditor.OnInspectorGUI () (at Library/PackageCache/com.unity.2d.spriteshape@2.1.0-preview.4/Editor/SpriteShapeEditor.cs:295)
UnityEditor.UIElements.InspectorElement+<CreateIMGUIInspectorFromEditor>c__AnonStorey1.<>m__0 () (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorElement.cs:479)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

5. If using an Angle Range, observe the default sprite cannot be unselected and the asset must be deleted to resolve errors