Steps to reproduce:
1. Open the provided project "case_1201281-Sandbox-Unity.zip"
2. In the Hierarchy window select "GameObject" GameObject
3. In the Inspector window try selecting any UnityEvent type function

Expected results: No errors are being thrown in the Console
Actual results: "type is not enum value" is being thrown in the Console

Reproduced with: 2020.1.0a15
Could not test with 2017.4, 2018.4, 2019.2, 2019.3 due to this issue being a new feature released in 2020.1.0a14

Error in the console:
type is not a enum value
UnityEditor.SerializedProperty:get_enumValueIndex()
UnityEditorInternal.UnityEventDrawer:GetMode(SerializedProperty)
UnityEditorInternal.UnityEventDrawer:DrawEvent(Rect, Int32, Boolean, Boolean)
UnityEditorInternal.ReorderableList:DoListElements(Rect)
UnityEditorInternal.ReorderableList:DoList(Rect)
UnityEditorInternal.UnityEventDrawer:OnGUI(Rect)
UnityEditorInternal.UnityEventDrawer:OnGUI(Rect, SerializedProperty, GUIContent)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)