Null fix for type converter.

backend/src/Squidex.Domain.Apps.Core.Operations/DefaultValues/DefaultValueGenerator.cs

@@ -64,7 +64,7 @@ namespace Squidex.Domain.Apps.Core.DefaultValues
                 return;
             }
 
-            if (!fieldData.TryGetValue(partitionKey, out var value))
+            if (!fieldData.ContainsKey(partitionKey))
             {
                 fieldData.AddLocalized(partitionKey, defaultValue);
             }

backend/src/Squidex.Domain.Apps.Entities.MongoDb/Contents/MongoContentRepository_SnapshotStore.cs

@@ -6,6 +6,7 @@
 // ==========================================================================
 
 using System;
+using System.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
 using Squidex.Domain.Apps.Core.Contents;
@@ -120,6 +121,10 @@ namespace Squidex.Domain.Apps.Entities.MongoDb.Contents
             {
                 content.ReferencedIds = content.Data.GetReferencedIds(schema.SchemaDef);
             }
+            else
+            {
+                content.ReferencedIds = new HashSet<DomainId>();
+            }
 
             return content;
         }

backend/src/Squidex.Infrastructure.MongoDb/MongoDb/TypeConverterStringSerializer.cs

@@ -34,15 +34,33 @@ namespace Squidex.Infrastructure.MongoDb
         }
 
         public override T Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
+        {
+            if (context.Reader.CurrentBsonType == BsonType.Null)
+            {
+                context.Reader.ReadNull();
+
+                return default!;
+            }
+            else
             {
                 var value = context.Reader.ReadString();
 
                 return (T)typeConverter.ConvertFromInvariantString(value);
             }
+        }
 
         public override void Serialize(BsonSerializationContext context, BsonSerializationArgs args, T value)
         {
-            context.Writer.WriteString(value!.ToString());
+            var text = value?.ToString();
+
+            if (text != null)
+            {
+                context.Writer.WriteString(text);
+            }
+            else
+            {
+                context.Writer.WriteNull();
+            }
         }
     }
 }

backend/tests/Squidex.Infrastructure.Tests/MongoDb/TypeConverterStringSerializerTests.cs

@@ -0,0 +1,112 @@
+// ==========================================================================
+//  Squidex Headless CMS
+// ==========================================================================
+//  Copyright (c) Squidex UG (haftungsbeschraenkt)
+//  All rights reserved. Licensed under the MIT license.
+// ==========================================================================
+
+using MongoDB.Bson;
+using MongoDB.Bson.IO;
+using MongoDB.Bson.Serialization;
+using Xunit;
+
+namespace Squidex.Infrastructure.MongoDb
+{
+    public class TypeConverterStringSerializerTests
+    {
+        public sealed record ValueHolder<T>
+        {
+            public T Value { get; set; }
+        }
+
+        public TypeConverterStringSerializerTests()
+        {
+            TypeConverterStringSerializer<DomainId>.Register();
+            TypeConverterStringSerializer<RefToken>.Register();
+        }
+
+        [Fact]
+        public void Should_serialize_struct()
+        {
+            var source = new ValueHolder<DomainId>
+            {
+                Value = DomainId.NewGuid()
+            };
+
+            var deserialized = SerializeAndDeserializeBson(source);
+
+            Assert.Equal(source, deserialized);
+        }
+
+        [Fact]
+        public void Should_serialize_nullable_struct()
+        {
+            var source = new ValueHolder<DomainId?>
+            {
+                Value = DomainId.NewGuid()
+            };
+
+            var deserialized = SerializeAndDeserializeBson(source);
+
+            Assert.Equal(source, deserialized);
+        }
+
+        [Fact]
+        public void Should_serialize_nullable_null_struct()
+        {
+            var source = new ValueHolder<DomainId?>
+            {
+                Value = null
+            };
+
+            var deserialized = SerializeAndDeserializeBson(source);
+
+            Assert.Equal(source, deserialized);
+        }
+
+        [Fact]
+        public void Should_serialize_class()
+        {
+            var source = new ValueHolder<RefToken>
+            {
+                Value = RefToken.Client("client")
+            };
+
+            var deserialized = SerializeAndDeserializeBson(source);
+
+            Assert.Equal(source, deserialized);
+        }
+
+        [Fact]
+        public void Should_serialize_null_class()
+        {
+            var source = new ValueHolder<RefToken?>
+            {
+                Value = null
+            };
+
+            var deserialized = SerializeAndDeserializeBson(source);
+
+            Assert.Equal(source, deserialized);
+        }
+
+        private static T SerializeAndDeserializeBson<T>(T value)
+        {
+            var document = new BsonDocument();
+
+            using (var writer = new BsonDocumentWriter(document))
+            {
+                BsonSerializer.Serialize(writer, value);
+
+                writer.Flush();
+            }
+
+            using (var reader = new BsonDocumentReader(document))
+            {
+                var result = BsonSerializer.Deserialize<T>(reader);
+
+                return result;
+            }
+        }
+    }
+}