AddUserSecrets is optional

src/Microsoft.AspNetCore/WebHost.cs

@@ -164,7 +164,7 @@ public static IWebHostBuilder CreateDefaultBuilder(string[] args)
                         var appAssembly = Assembly.Load(new AssemblyName(env.ApplicationName));
                         if (appAssembly != null)
                         {
-                            config.AddUserSecrets(appAssembly);
+                            config.AddUserSecrets(appAssembly, optional: true);
                         }
                     }