Исправление

SelfPropelledArtilleryUnit/FormArtilleryConfig.cs

@@ -118,6 +118,5 @@ namespace Artilleries
             EventAddArtillery?.Invoke(_artillery);
             Close();
         }
-
     }
 }

SelfPropelledArtilleryUnit/FormMapWithSetArtilleries.cs

@@ -89,7 +89,12 @@ namespace Artilleries
         private void buttonAddArtillery_Click(object sender, EventArgs e)
         {
             var formArtilleryConfig = new FormArtilleryConfig();
-            formArtilleryConfig.AddEvent((artillery) => { 
+            formArtilleryConfig.AddEvent((artillery) => {
+                if (listBoxMaps.SelectedIndex == -1)
+                {
+                    return;
+                }
+
                 if (_mapsCollection[listBoxMaps.SelectedItem?.ToString() ?? String.Empty] + new DrawingObjectArtillery(artillery) != -1)
                 {
                     MessageBox.Show("Объект добавлен");