New MMTConstant; BugFix: Nullpointer; Lucas Video;

Assets/Scripts/InventoryStuff/DisplayScrolls.cs

@@ -42,6 +42,6 @@ void BuildScrollGUI()
 
         REST_JSON_API.Scroll preferredStartScroll = AllowedScrolls.Find(x => x.label.Equals(preferredStartScrollName));
         if (preferredStartScroll != null)
-            ScrollDetails.Instance.SetScroll(preferredStartScroll);
+            ScrollDetails.Instance?.SetScroll(preferredStartScroll);
     }
 }

Assets/Scripts/MMTServer/CommunicationProtocoll/MMTConstants.cs

@@ -80,6 +80,7 @@ public static class MMTConstants
         public static readonly string ListEnd = "http://gl.mathhub.info/MMT/LFX/Datatypes?ListSymbols?nil_constant";
 
         public static readonly string AddRealLit = "http://mathhub.info/MitM/Foundation?RealLiterals?plus_real_lit";
+        public static readonly string AddArith = "http://mathhub.info/MitM/core/arithmetics?RealArithmetics?addition";
         public static readonly string MinusRealLit = "http://mathhub.info/MitM/Foundation?RealLiterals?minus_real_lit";
         public static readonly string InvertRealLit = "http://mathhub.info/MitM/Foundation?RealLiterals?inv_real_lit";
         public static readonly string TimesRealLit = "http://mathhub.info/MitM/Foundation?RealLiterals?times_real_lit";

Assets/Scripts/MMTServer/CommunicationProtocoll/SOMDocToLambdaExpression.cs

@@ -183,6 +183,8 @@ protected static class SOMDocToLambdaExpression
         {
             { MMTConstants.AddRealLit,
                 ExpressionType.Add},
+            { MMTConstants.AddArith,
+                ExpressionType.Add},
             { MMTConstants.PointAddI,
                 ExpressionType.Add},
             { "AddAssign",