Can't assign a prefab to PlayerObjectAssigner

[TyroneSama]

Unity 2019.4.31f1, UdonSharp v0.20.3—UI seems broken. Bringing up the Inspector for either PlayerObjectAssigner or PlayerObjectPool produces this error:

NullReferenceException: Object reference not set to an instance of an object
UdonSharpEditor.UdonSharpComponentExtensions.ConvertToUdonSharpComponentIntnl (VRC.Udon.UdonBehaviour behaviour, System.Type type, UdonSharpEditor.ProxySerializationPolicy proxySerializationPolicy) (at Assets/UdonSharp/Editor/Editors/UdonSharpComponentExtensions.cs:68)
UdonSharpEditor.UdonSharpComponentExtensions.ConvertToUdonSharpComponents[T] (VRC.Udon.UdonBehaviour[] behaviours, UdonSharpEditor.ProxySerializationPolicy proxySerializationPolicy) (at Assets/UdonSharp/Editor/Editors/UdonSharpComponentExtensions.cs:124)
UdonSharpEditor.UdonSharpComponentExtensions.GetUdonSharpComponentsInChildren[T] (UnityEngine.GameObject gameObject, System.Boolean includeInactive) (at Assets/UdonSharp/Editor/Editors/UdonSharpComponentExtensions.cs:339)
Cyan.PlayerObjectPool.CyanPoolSetupHelper.<GetPoolUdon>g__GetPoolUdon|5_0 (UnityEngine.GameObject obj) (at Assets/Cyan/PlayerObjectPool/Scripts/CyanPoolSetupHelper.cs:81)
Cyan.PlayerObjectPool.CyanPoolSetupHelper.GetPoolUdon () (at Assets/Cyan/PlayerObjectPool/Scripts/CyanPoolSetupHelper.cs:94)
Cyan.PlayerObjectPool.CyanPoolSetupHelper.GetPoolSize () (at Assets/Cyan/PlayerObjectPool/Scripts/CyanPoolSetupHelper.cs:117)
Cyan.PlayerObjectPool.CyanPlayerObjectAssignerEditor.RenderHelperSettings () (at Assets/Cyan/PlayerObjectPool/Scripts/Editor/CyanPlayerObjectAssignerEditor.cs:101)
Cyan.PlayerObjectPool.CyanPlayerObjectAssignerEditor.OnInspectorGUI () (at Assets/Cyan/PlayerObjectPool/Scripts/Editor/CyanPlayerObjectAssignerEditor.cs:49)
UdonSharpEditor.UdonBehaviourOverrideEditor.OnInspectorGUI () (at Assets/UdonSharp/Editor/Editors/UdonSharpBehaviourEditor.cs:507)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass58_0.<CreateIMGUIInspectorFromEditor>b__0 () (at <a259d3c004024353a2c217da97495055>:0)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

I assume this is dumb user error somehow, but not really sure how to proceed—I've tried reimporting UdonSharp just to be sure, but no luck.