Added debug message is runtime set does not exist

Scripts/Targetable.cs

@@ -19,7 +19,14 @@ namespace GameDev.CoreSystems
 
         private void OnEnable()
         {
-            _activeTargetableRuntimeSet.Add(this);
+            if (_activeTargetableRuntimeSet != null)
+            {
+                _activeTargetableRuntimeSet.Add(this);
+            }
+            else
+            {
+                Debug.LogError("TargetableRuntimeSet does not exist");
+            }
         }
 
         private void OnDisable()