How Do I Ensure that --enable-unity-event-support is Set for iOS Builds?

Hey all, I've been getting NullReferenceExceptions for my IL2CPP builds whenever I call AddListener() on my derived UnityEvent. Here is my UnityEvent derivative:

 [Serializable]
 public class TweenEvent : UnityEvent<Tween> { }

Here is the code that's getting called:

 // setup the in-Tween
 _inTween = gameObject.AddComponent<PositionTween>();
 _inTween.TweenFinished.AddListener(HandleTransitionInTweenFinished); // exception occurs
 _inTween.Curve = new AnimationCurve(EaseInKeyframes);
 _inTween.Begin = new Vector3(TransitionDistance, 0.0f, 0.0f);
 _inTween.End = Vector3.zero;
 _inTween.Duration = TransitionDuration;

The NullReferenceException happens with AddListener(), but that's all the information I get. I've verified that _inTween and _inTween's TweenFinished property (TweenEvent) are not null using debug logs. I get no runtime exceptions or have any issues with my code in the editor or with builds on Android devices. Everything runs fine on iOS when I use Mono2x instead of IL2CPP. I need to use IL2CPP though.

I'm also using Unity 5.1.2f on OS X El Capitan. I'm not able to upgrade my version of Unity due to our internal framework's compatibility managed by another team.

Also, is there a way that I can verify that --enable-unity-event-support is being passed to il2cpp when making iOS builds from OS X?

UPDATE: I did a test with C# delegates instead of UnityEvents, and I got the same error. Here's the error I get in Xcode:

 NullReferenceException: A null value was found where an object instance was required.
   at Step.Setup (Controller controller, UnityEngine.RectTransform spawnTransform) [0x00000] in <filename unknown>:0 
   at Controller+<ProcessLoadStep>c__Iterator9.MoveNext () [0x00000] in <filename unknown>:0 

It still happens at the AddListener() line. Here's the complete Setup() method that I'm having trouble with:

 public void Setup(Controller controller, RectTransform spawnTransform)
     {
         // get the RectTransform, and assign the Parent controller
         RectTransform rectTransform = (RectTransform)transform;
         Parent = controller;
         if(Parent != null)
         {
             // setup the RectTransform
             rectTransform.SetParent(spawnTransform);
             rectTransform.offsetMin = Vector2.zero;
             rectTransform.offsetMax = Vector2.one;
             rectTransform.localScale = Vector3.one;
                 
             // setup the in-Tween
             _inTween = gameObject.AddComponent<PositionTween>();
             _inTween.TweenFinished.AddListener(HandleTransitionInTweenFinished); // EXCEPTION
             _inTween.Curve = new AnimationCurve(EaseInKeyframes);
             _inTween.Begin = new Vector3(TransitionDistance, 0.0f, 0.0f);
             _inTween.End = Vector3.zero;
             _inTween.Duration = TransitionDuration;
                 
             // setup the out-Tween
             _outTween = gameObject.AddComponent<PositionTween>();
             _outTween.TweenFinished.AddListener(HandleTransitionOutTweenFinished); // EXCEPTION
             _outTween.Curve = new AnimationCurve(EaseInKeyframes);
             _outTween.Begin = Vector3.zero;
             _outTween.End = new Vector3(-TransitionDistance, 0.0f, 0.0f);
             _outTween.Duration = TransitionDuration;
         }
     }

This suggests that my private HandleTransitionIn(Out)TweenFinished() methods are null, but how can that be? The GameObject and component instance are both created. I also get more NullReferenceExceptions through the runtime of my game on iOS/IL2CPP that never occurs on iOS/Mono or Android builds.