diff --git a/AirFighter/AirFighter/FormMilitaryAircraftConfig.cs b/AirFighter/AirFighter/FormMilitaryAircraftConfig.cs
index 9653392..4e319a6 100644
--- a/AirFighter/AirFighter/FormMilitaryAircraftConfig.cs
+++ b/AirFighter/AirFighter/FormMilitaryAircraftConfig.cs
@@ -41,7 +41,14 @@ public partial class FormMilitaryAircraftConfig : Form
     /// <param name="militaryAircraftDelegate"></param>
     public void AddEvent(Action<DrawningMilitaryAircraft> militaryAircraftDelegate)
     {
-        MilitaryAircraftDelegate += militaryAircraftDelegate;
+        if (MilitaryAircraftDelegate != null)
+        {
+            MilitaryAircraftDelegate = militaryAircraftDelegate;
+        }
+        else
+        {
+            MilitaryAircraftDelegate += militaryAircraftDelegate;
+        }
     }
 
     /// <summary>