Event throwing null reference exception in OnTriggerEnter

I'm trying to create a means so that when something in my game takes damage, it can notify any other scripts that need to know - I'm doing this by means of an event, which the other scripts hook up to:

 using UnityEngine;
 using System;
 
 namespace Common
 {
     public class DamageCollisionListener : MonoBehaviour
     {
         public event Action<Transform> TriggerEntryEvent;
 
         public void OnTriggerEnter(Collider collidedWith)
         {
             if (collidedWith.tag == GlobalConstants.Damage_Collider_Tag)
             {
                 Debug.Log(collidedWith.name + " was hit, fire event (I am " + transform.name + ")");
                 TriggerEntryEvent.Invoke(collidedWith.transform);
             }
         }
     }
 }

One of the classes that hook up to this is meant to update a "state" value when a hit is taken (this is currently part-finished, I'm trying to test the event hook-up):

 using UnityEngine;
 using Common;
 using Characters.Shared.Foundation.Interfaces;
 
 namespace Characters.Shared.StateUpdateTriggers
 {
     public class TakeHitStateTrigger : IStateUpdateTrigger
     {
         private DamageCollisionListener _damageListener;
         private Transform _characterTransform;
 
         public TakeHitStateTrigger(DamageCollisionListener damageListener, Transform characterTransform)
         {
             _characterTransform = characterTransform;
 
             _damageListener = damageListener;
             _damageListener.TriggerEntryEvent += HandleTriggerEntryEvent;
         }
 
         private void HandleTriggerEntryEvent(Transform hitByTransform)
         {
             Debug.Log("HIT");
         }
 
         public CharacterState HandleStateUpdate(CharacterState state)
         {
             return state;
         }
     }
 }

... but instead of the expected "HIT" being logged in the console when the trigger collision fires, I am getting a null reference exception.

I have tested this quite exhaustively: I added in a bit of functionality to force the event to be invoked directly after wire-up in the TakeHitStateTrigger - the event fired perfectly. I then tried adding the same forced invocation in the TakeHitStateTrigger.HandleStateUpdate method (which is called every update) - again, the event fires as expected. It is just when the actual OnTriggerEnter occurs that the event fails.

Any ideas would be useful - this is driving me nuts!

Thanks!