Merge pull request #5876 from Bond-009/nullref5

DeepCopy: Throw ArgumentNullException if one of the args is null

MediaBrowser.Controller/Entities/BaseItemExtensions.cs

@@ -1,5 +1,7 @@
+#nullable enable
 #pragma warning disable CS1591
 
+using System;
 using System.Linq;
 using MediaBrowser.Model.Entities;
 using MediaBrowser.Model.IO;
@@ -67,6 +69,16 @@ namespace MediaBrowser.Controller.Entities
             where T : BaseItem
             where TU : BaseItem
         {
+            if (source == null)
+            {
+                throw new ArgumentNullException(nameof(source));
+            }
+
+            if (dest == null)
+            {
+                throw new ArgumentNullException(nameof(dest));
+            }
+
             var destProps = typeof(TU).GetProperties().Where(x => x.CanWrite).ToList();
 
             foreach (var sourceProp in typeof(T).GetProperties())