InvalidOperationException when peeking state in custom game editor

I've got a simple state machine for handling game states which works using GameObjects that I enable/disable in order to switch the states. Since some states can be loaded additively (such as the pause menu), I'm using a stack to manage the currently active ones.

 public class GameController : MonoBehavior {

     private readonly Stack<GameObject> _stateStack = new Stack<GameObject>();

     public GameObject ActiveState {
         get { return _stateStack.Peek(); }
     }

     public void SetState(GameObject newState, bool isAdditive = false) {
         if(newState = null || newState == ActiveState)
             return;

         if(!isAdditive) {
             while(_stateStack.Count > 0)
                 PopState();
         }

         if(!newState.activeInHierarchy)
             newState.SetActive(true);

         _stateStack.Push(newState);
     }

     public void PopState() {
         var gameState = _statStack.Pop();
         if(gameState != null)
             gameState.SetActive(false);
     }

 }

Now I'd like to have a custom editor which allows me to see and set the currently top-most active state.

What I did was this:

 [CustomEditor(typeof(GameController))]
 public class GameControllerEditor : Editor {

     public override void OnInspectorGUI() {
         DrawDefaultInspector();

         var controller = target as GameController;
         if(controller == null)
             return;

         var newState = EditorGUILayout.ObjectField("ActiveState", controller.ActiveState, 
               typeof (GameObject), true) as GameObject;
         if (newState != controller.ActiveState)
             controller.SetState(newState);
     }

 }

However, the custom editor is never visible and in the console, I see the following exception:

 InvalidOperationException: Operation is not valid due to the current state of the object
 System.Collections.Generic.Stack'1[UnityEngine.GameObject].Peek()

What does this even mean? What state? Is the stack not initialized? How does Unity instantiate MonoBehaviors when applying a custom inspector?