Added debug message is runtime set does not exist

Scripts/Targetable.cs

@@ -18,9 +18,16 @@ namespace GameDev.CoreSystems
         [field: SerializeField] public Collider Collider { get; private set; }
 
         private void OnEnable()
+        {
+            if (_activeTargetableRuntimeSet != null)
             {
                 _activeTargetableRuntimeSet.Add(this);
             }
+            else
+            {
+                Debug.LogError("TargetableRuntimeSet does not exist");
+            }
+        }
 
         private void OnDisable()
         {