Migrations - Fixed color gradient migration issue

src/Artemis.Storage/Migrations/M0011ColorGradients.cs

@@ -12,7 +12,11 @@ namespace Artemis.Storage.Migrations
 
             foreach (BsonValue propertyEntity in propertyEntities.AsArray)
             {
+                if (propertyEntity["Value"] == null)
+                    continue;
                 string valueString = propertyEntity["Value"].AsString;
+                if (valueString == null)
+                    continue;
                 if (!valueString.StartsWith("{\"Stops\":[{") || !valueString.EndsWith("}]}"))
                     continue;