NullReferenceException: Object reference not set to an instance of an object
UnityEditorInternal.UnityEventDrawer.BuildPopupList (UnityEngine.Object target, UnityEngine.Events.UnityEventBase dummyEvent, UnityEditor.SerializedProperty listener) (at <ad50112e30b445fdbf80d5814c3cbcf6>:0)
UnityEditorInternal.UnityEventDrawer.DrawEvent (UnityEngine.Rect rect, System.Int32 index, System.Boolean isActive, System.Boolean isFocused) (at <ad50112e30b445fdbf80d5814c3cbcf6>:0)
UnityEditorInternal.ReorderableList.DoListElements (UnityEngine.Rect listRect, UnityEngine.Rect visibleRect) (at <ad50112e30b445fdbf80d5814c3cbcf6>:0)
UnityEditorInternal.ReorderableList.DoList (UnityEngine.Rect rect, UnityEngine.Rect visibleRect) (at <ad50112e30b445fdbf80d5814c3cbcf6>:0)
UnityEditorInternal.ReorderableList.DoList (UnityEngine.Rect rect) (at <ad50112e30b445fdbf80d5814c3cbcf6>:0)
UnityEditorInternal.UnityEventDrawer.OnGUI (UnityEngine.Rect position) (at <ad50112e30b445fdbf80d5814c3cbcf6>:0)
UnityEditorInternal.UnityEventDrawer.OnGUI (UnityEngine.Rect position, UnityEditor.SerializedProperty property, UnityEngine.GUIContent label) (at <ad50112e30b445fdbf80d5814c3cbcf6>:0)
UnityEditor.PropertyDrawer.OnGUISafe (UnityEngine.Rect position, UnityEditor.SerializedProperty property, UnityEngine.GUIContent label) (at <ad50112e30b445fdbf80d5814c3cbcf6>:0)
UnityEditor.PropertyHandler.OnGUI (UnityEngine.Rect position, UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren, UnityEngine.Rect visibleArea) (at <ad50112e30b445fdbf80d5814c3cbcf6>:0)
UnityEditor.PropertyHandler.OnGUI (UnityEngine.Rect position, UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren) (at <ad50112e30b445fdbf80d5814c3cbcf6>:0)
UnityEditor.PropertyHandler.OnGUILayout (UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren, UnityEngine.GUILayoutOption[] options) (at <ad50112e30b445fdbf80d5814c3cbcf6>:0)
UnityEditor.EditorGUILayout.PropertyField (UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren, UnityEngine.GUILayoutOption[] options) (at <ad50112e30b445fdbf80d5814c3cbcf6>:0)
UnityEditor.EditorGUILayout.PropertyField (UnityEditor.SerializedProperty property, UnityEngine.GUILayoutOption[] options) (at <ad50112e30b445fdbf80d5814c3cbcf6>:0)
UnityEditor.UI.ButtonEditor.OnInspectorGUI () (at Library/PackageCache/com.unity.ugui@1.0.0/Editor/UI/ButtonEditor.cs:27)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass59_0.<CreateIMGUIInspectorFromEditor>b__0 () (at <1ada6c7052bb42378c5ec1bd01fc4723>:0)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr, Boolean&)