Tutorial fixes

Subsurface/Source/Map/Levels/Level.cs

@@ -76,7 +76,10 @@ namespace Subsurface
 
         public Vector2 Position
         {
-            get { return ConvertUnits.ToDisplayUnits(cells[0].body.Position); }
+            get 
+            {                 
+                return cells==null ? Vector2.Zero : ConvertUnits.ToDisplayUnits(cells[0].body.Position); 
+            }
         }
 
         public List<Vector2> PositionsOfInterest

Subsurface/Source/Map/Submarine.cs

@@ -623,11 +623,13 @@ namespace Subsurface
             loaded = this;
         }
 
-        public static Submarine Load(string fileName)
+        public static Submarine Load(string fileName, string folder = SavePath)
         {
-            Unload();            
+            Unload();
 
-            Submarine sub = new Submarine(SavePath+"/"+fileName);
+            string path = string.IsNullOrWhiteSpace(folder) ? fileName : System.IO.Path.Combine(SavePath, fileName);
+
+            Submarine sub = new Submarine(path);
             sub.Load();
 
             //Entity.dictionary.Add(int.MaxValue, sub);