CheckComboBox: Fix SelectedItems property

Tests/E2ETests/MainWindowTest.cs

@@ -46,5 +46,16 @@
                 window.Close();
             });
         }
+
+        [Test]
+        public void Test_Open_MailWindow() {
+            Assert.DoesNotThrow(() => {
+                Session.App.FindElement(By.Name("Rundschreiben")).Click();
+                Thread.Sleep(Utils.WINDOW_OPEN_SLEEP);
+                var window = Session.CreateWindowDriver("MailWindow");
+                Assert.That(window.Title, Is.EqualTo($"Rundschreiben - Lese {Elwig.Helpers.Utils.Today.Year} - Elwig"));
+                window.Close();
+            });
+        }
     }
 }