Fix null pointer when saving selectors

Author: cesarBLG

Source/Orts.Simulation/Simulation/RollingStocks/SubSystems/PowerSupplies/Controllers/PantographSelector.cs

@@ -146,13 +146,13 @@ public void InitializeMoving()
 
         public void Save(BinaryWriter outf)
         {
-            outf.Write(Position?.Name);
+            outf.Write(Position?.Name ?? string.Empty);
         }
 
         public void Restore(BinaryReader inf)
         {
             string name = inf.ReadString();
-            Position = Positions.FirstOrDefault(x => x.Name == name) ?? Positions.FirstOrDefault();
+            Position = Positions.FirstOrDefault(x => x.Name == name);
         }
 
         public void Update(float elapsedClockSeconds)

Source/Orts.Simulation/Simulation/RollingStocks/SubSystems/PowerSupplies/Controllers/PowerLimitationSelector.cs

@@ -148,13 +148,13 @@ public void InitializeMoving()
 
         public void Save(BinaryWriter outf)
         {
-            outf.Write(Position?.Name);
+            outf.Write(Position?.Name ?? string.Empty);
         }
 
         public void Restore(BinaryReader inf)
         {
             string name = inf.ReadString();
-            Position = Positions.FirstOrDefault(x => x.Name == name) ?? Positions.FirstOrDefault();
+            Position = Positions.FirstOrDefault(x => x.Name == name);
         }
 
         public void Update(float elapsedClockSeconds)

Source/Orts.Simulation/Simulation/RollingStocks/SubSystems/PowerSupplies/Controllers/VoltageSelector.cs

@@ -149,13 +149,13 @@ public void InitializeMoving()
 
         public void Save(BinaryWriter outf)
         {
-            outf.Write(Position?.Name);
+            outf.Write(Position?.Name ?? string.Empty);
         }
 
         public void Restore(BinaryReader inf)
         {
             string name = inf.ReadString();
-            Position = Positions.FirstOrDefault(x => x.Name == name) ?? Positions.FirstOrDefault();
+            Position = Positions.FirstOrDefault(x => x.Name == name);
         }
 
         public void Update(float elapsedClockSeconds)