only allow reference types on the server side

Pilz.Net/Api/Server/BaseHandler.cs

@@ -2,15 +2,12 @@
 using Pilz.Net.Data;
 using Pilz.Net.Extensions;
 using System.Diagnostics;
-using System.Linq.Expressions;
-using System.Reflection;
-using System.Runtime.CompilerServices;
 
 namespace Pilz.Net.Api.Server;
 
 public abstract class BaseHandler<TEntity, TUpdateMsg>(IApiServer server)
     : IApiHandlerInitializer
-    where TEntity : IDataObject
+    where TEntity : class, IDataObject
     where TUpdateMsg : ApiMessage
 {
     public abstract string Route { get; }