Fix: F9 crashes with a front coupled single steam locomotive.

Author: mbm-OR

Source/RunActivity/Viewer3D/Popups/TrainCarOperationsViewerWindow.cs

@@ -605,8 +605,12 @@ public buttonCouplerFront(int x, int y, int size, Viewer viewer, TrainCar car, i
                 bool first = car == Viewer.PlayerTrain.Cars.First();
                 var CurrentCar = Viewer.PlayerTrain.Cars[carPosition];
 
-                var isSteamAndHasTender = (CurrentCar is MSTSSteamLocomotive) &&
-                    (carPosition + (CurrentCar.Flipped ? -1 : 1) < Viewer.PlayerTrain.Cars.Count) && (Viewer.PlayerTrain.Cars[carPosition + (CurrentCar.Flipped ? -1 : 1)].WagonType == MSTSWagon.WagonTypes.Tender);
+                var isSteamAndHasTender = false;
+                if (CurrentCar is MSTSSteamLocomotive)
+                {
+                    var validTenderPosition = CurrentCar.Flipped ? carPosition - 1 > -1 : carPosition + 1 < Viewer.PlayerTrain.Cars.Count;
+                    isSteamAndHasTender = validTenderPosition && (Viewer.PlayerTrain.Cars[carPosition + (CurrentCar.Flipped ? -1 : 1)].WagonType == MSTSWagon.WagonTypes.Tender);
+                }
                 var isTender = CurrentCar.WagonType == MSTSWagon.WagonTypes.Tender;
                 if (isSteamAndHasTender || isTender)
                 {

Source/RunActivity/Viewer3D/Popups/TrainCarOperationsWindow.cs

@@ -915,8 +915,12 @@ public buttonCouplerFront(int x, int y, int size, Viewer viewer, TrainCar car, i
             First = car == Viewer.PlayerTrain.Cars.First();
             var CurrentCar = Viewer.PlayerTrain.Cars[carPosition]; ;
 
-            var isSteamAndHasTender = (CurrentCar is MSTSSteamLocomotive) &&
-                (carPosition + (CurrentCar.Flipped ? -1 : 1) < Viewer.PlayerTrain.Cars.Count) && (Viewer.PlayerTrain.Cars[carPosition + (CurrentCar.Flipped ? -1 : 1)].WagonType == MSTSWagon.WagonTypes.Tender);
+            var isSteamAndHasTender = false;
+            if (CurrentCar is MSTSSteamLocomotive)
+            {
+                var validTenderPosition = CurrentCar.Flipped ? carPosition - 1 > -1 : carPosition + 1 < Viewer.PlayerTrain.Cars.Count;
+                isSteamAndHasTender = validTenderPosition && (Viewer.PlayerTrain.Cars[carPosition + (CurrentCar.Flipped ? -1 : 1)].WagonType == MSTSWagon.WagonTypes.Tender);
+            }
             var isTender = CurrentCar.WagonType == MSTSWagon.WagonTypes.Tender;
 
             if (isSteamAndHasTender || isTender)