UnityEvent NullreferenceException in inspector

I'm getting an error whenever I try to add an Event to a UnityEvent field in the inspector (clicking on the plus symbol in the corner). Whenever I hit the I'm running Unity 5.5.0f3 and have tried restarting Unity several times. It's not happening to all UnityEvents I'm using, just these ones, that are in a serializable class rather deep in the object hierarchy. Could the problem be that the definition is too deep in the object hierarchy? I have a Monobehaviour that has a serializable-class variable, that has a list of another serializable-class variable that contains a list of type customEvent.

gif version

The code:

 [System.Serializable]
 public class customEvent
 {
 
 
 
     public List<GameEvent> events;
     public UnityEvent selfdefinedEvents;
  
 
     public void Invoke(room_controller rooms)
     {
         foreach (GameEvent ge in events)
         {
 
             ge.Invoke(rooms);
 
         }
 
         selfdefinedEvents.Invoke();
 
     }
 }
 

The error: NullReferenceException: Object reference not set to an instance of an object UnityEditorInternal.UnityEventDrawer.AddEventListener (UnityEditorInternal.ReorderableList list) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/UnityEventDrawer.cs:365) UnityEditorInternal.ReorderableList+Defaults.DrawFooter (Rect rect, UnityEditorInternal.ReorderableList list) (at /Users/builduser/buildslave/unity/build/Editor/Mono/GUI/ReorderableList.cs:105) UnityEditorInternal.ReorderableList.DoListFooter (Rect footerRect) (at /Users/builduser/buildslave/unity/build/Editor/Mono/GUI/ReorderableList.cs:627) UnityEditorInternal.ReorderableList.DoList (Rect rect) (at /Users/builduser/buildslave/unity/build/Editor/Mono/GUI/ReorderableList.cs:400) UnityEditorInternal.UnityEventDrawer.OnGUI (Rect position) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/UnityEventDrawer.cs:157) UnityEditorInternal.UnityEventDrawer.OnGUI (Rect position, UnityEditor.SerializedProperty property, UnityEngine.GUIContent label) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/UnityEventDrawer.cs:123) UnityEditor.PropertyDrawer.OnGUISafe (Rect position, UnityEditor.SerializedProperty property, UnityEngine.GUIContent label) (at /Users/builduser/buildslave/unity/build/Editor/Mono/ScriptAttributeGUI/PropertyDrawer.cs:23) UnityEditor.PropertyHandler.OnGUI (Rect position, UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, Boolean includeChildren) (at /Users/builduser/buildslave/unity/build/Editor/Mono/ScriptAttributeGUI/PropertyHandler.cs:134) UnityEditor.EditorGUI.PropertyFieldInternal (Rect position, UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, Boolean includeChildren) (at /Users/builduser/buildslave/unity/build/Editor/Mono/EditorGUI.cs:5158) UnityEditor.EditorGUI.PropertyField (Rect position, UnityEditor.SerializedProperty property, Boolean includeChildren) (at /Users/builduser/buildslave/unity/build/artifacts/generated/common/editor/EditorGUIBindings.gen.cs:1041) UnityEditor.EditorGUI.PropertyField (Rect position, UnityEditor.SerializedProperty property) (at /Users/builduser/buildslave/unity/build/artifacts/generated/common/editor/EditorGUIBindings.gen.cs:1036) UnityEditor.Editor.OptimizedInspectorGUIImplementation (Rect contentRect) (at /Users/builduser/buildslave/unity/build/artifacts/generated/common/editor/EditorBindings.gen.cs:233) UnityEditor.GenericInspector.OnOptimizedInspectorGUI (Rect contentRect) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/GenericInspector.cs:33) UnityEditor.InspectorWindow.DrawEditor (UnityEditor.Editor editor, Int32 editorIndex, Boolean rebuildOptimizedGUIBlock, System.Boolean& showImportedObjectBarNext, UnityEngine.Rect& importedObjectBarRect) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1216) UnityEditor.InspectorWindow.DrawEditors (UnityEditor.Editor[] editors) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1031) UnityEditor.InspectorWindow.OnGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:356) System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222)