Fixes: U4-6733 Adding dictionary item (7.3): No parent dictionary item was found with id 41c7638d-f529-4bff-853e-59a0c2fb1bde

src/umbraco.cms/businesslogic/Dictionary.cs

@@ -325,7 +325,9 @@ namespace umbraco.cms.businesslogic
                 if (!hasKey(key))
                 {
                     var item = ApplicationContext.Current.Services.LocalizationService.CreateDictionaryItemWithIdentity(
-                        key, parentId, defaultValue);
+                        key,
+                        parentId == TopLevelParent ? (Guid?)null : parentId, 
+                        defaultValue);
 
                     return item.Id;
                 }