Update dev

coscine/issues#2254

src/Tree/Controllers/TreeController.cs

@@ -192,7 +192,7 @@ namespace Coscine.Api.Tree.Controllers
                                         graph.Assert(
                                             value.Subject,
                                             value.Predicate,
-                                            graph.CreateLiteralNode((value.Object as LiteralNode).Value, new Uri(shDatatype.Object.ToString()))
+                                            graph.CreateLiteralNode((value.Object as LiteralNode)?.Value, new Uri(shDatatype.Object.ToString()))
                                         );
                                     }
                                 }
@@ -424,7 +424,7 @@ namespace Coscine.Api.Tree.Controllers
                 var tripleObject = triple.Object;
                 if (tripleObject is ILiteralNode)
                 {
-                    var tripleObjectString = (tripleObject as ILiteralNode).Value;
+                    var tripleObjectString = (tripleObject as ILiteralNode)?.Value;
                     if (tripleObjectString.Equals("{ME}"))
                     {
                         tripleObjectString = tripleObjectString.Replace("{ME}", user.DisplayName);