Merge pull request #755 from Csantucci/throttle-air-combined-control-fix

Bug fix for https://bugs.launchpad.net/or/+bug/1999330 Crash with a throttle-air combined control

Author: Csantucci

Source/Orts.Simulation/Simulation/RollingStocks/MSTSLocomotive.cs

@@ -3951,7 +3951,7 @@ public void SetCombinedHandleValue(float value)
                 else if (CombinedControlType == CombinedControl.ThrottleAir && canBrake && value > CombinedControlSplitPosition)
                     SetTrainBrakeValue((MathHelper.Clamp(value, CombinedControlSplitPosition, 1) - CombinedControlSplitPosition) / (1 - CombinedControlSplitPosition));
                 else if (DynamicBrakePercent < 0 || TrainControlSystem.FullDynamicBrakingOrder ||
-                    (!CruiseControl.DynamicBrakePriority && CruiseControl.SpeedRegMode == CruiseControl.SpeedRegulatorMode.Auto))
+                    (CruiseControl != null && !CruiseControl.DynamicBrakePriority && CruiseControl.SpeedRegMode == CruiseControl.SpeedRegulatorMode.Auto))
                     SetThrottleValue(1 - MathHelper.Clamp(value, 0, CombinedControlSplitPosition) / CombinedControlSplitPosition);
             }
         }