diff --git a/backend/src/Squidex.Domain.Apps.Core.Model/Schemas/IFieldVisitor.cs b/backend/src/Squidex.Domain.Apps.Core.Model/Schemas/IFieldVisitor.cs
index 0b2340cab..710560f6d 100644
--- a/backend/src/Squidex.Domain.Apps.Core.Model/Schemas/IFieldVisitor.cs
+++ b/backend/src/Squidex.Domain.Apps.Core.Model/Schemas/IFieldVisitor.cs
@@ -5,65 +5,30 @@
 //  All rights reserved. Licensed under the MIT license.
 // ==========================================================================
 
-#pragma warning disable CS8653 // A default expression introduces a null value for a type parameter.
-
 namespace Squidex.Domain.Apps.Core.Schemas
 {
     public interface IFieldVisitor<out T>
     {
-        T Visit(IArrayField field)
-        {
-            return default;
-        }
+        T Visit(IArrayField field);
 
-        T Visit(IField<AssetsFieldProperties> field)
-        {
-            return default;
-        }
+        T Visit(IField<AssetsFieldProperties> field);
 
-        T Visit(IField<BooleanFieldProperties> field)
-        {
-            return default;
-        }
+        T Visit(IField<BooleanFieldProperties> field);
 
-        T Visit(IField<DateTimeFieldProperties> field)
-        {
-            return default;
-        }
+        T Visit(IField<DateTimeFieldProperties> field);
 
-        T Visit(IField<GeolocationFieldProperties> field)
-        {
-            return default;
-        }
+        T Visit(IField<GeolocationFieldProperties> field);
 
-        T Visit(IField<JsonFieldProperties> field)
-        {
-            return default;
-        }
+        T Visit(IField<JsonFieldProperties> field);
 
-        T Visit(IField<NumberFieldProperties> field)
-        {
-            return default;
-        }
+        T Visit(IField<NumberFieldProperties> field);
 
-        T Visit(IField<ReferencesFieldProperties> field)
-        {
-            return default;
-        }
+        T Visit(IField<ReferencesFieldProperties> field);
 
-        T Visit(IField<StringFieldProperties> field)
-        {
-            return default;
-        }
+        T Visit(IField<StringFieldProperties> field);
 
-        T Visit(IField<TagsFieldProperties> field)
-        {
-            return default;
-        }
+        T Visit(IField<TagsFieldProperties> field);
 
-        T Visit(IField<UIFieldProperties> field)
-        {
-            return default;
-        }
+        T Visit(IField<UIFieldProperties> field);
     }
 }
diff --git a/backend/src/Squidex.Domain.Apps.Core.Operations/ExtractReferenceIds/ReferencesExtractor.cs b/backend/src/Squidex.Domain.Apps.Core.Operations/ExtractReferenceIds/ReferencesExtractor.cs
index 3a2f0fc81..5a7801685 100644
--- a/backend/src/Squidex.Domain.Apps.Core.Operations/ExtractReferenceIds/ReferencesExtractor.cs
+++ b/backend/src/Squidex.Domain.Apps.Core.Operations/ExtractReferenceIds/ReferencesExtractor.cs
@@ -70,5 +70,45 @@ namespace Squidex.Domain.Apps.Core.ExtractReferenceIds
 
             return None.Value;
         }
+
+        public None Visit(IField<BooleanFieldProperties> field)
+        {
+            return None.Value;
+        }
+
+        public None Visit(IField<DateTimeFieldProperties> field)
+        {
+            return None.Value;
+        }
+
+        public None Visit(IField<GeolocationFieldProperties> field)
+        {
+            return None.Value;
+        }
+
+        public None Visit(IField<JsonFieldProperties> field)
+        {
+            return None.Value;
+        }
+
+        public None Visit(IField<NumberFieldProperties> field)
+        {
+            return None.Value;
+        }
+
+        public None Visit(IField<StringFieldProperties> field)
+        {
+            return None.Value;
+        }
+
+        public None Visit(IField<TagsFieldProperties> field)
+        {
+            return None.Value;
+        }
+
+        public None Visit(IField<UIFieldProperties> field)
+        {
+            return None.Value;
+        }
     }
 }