[fix:] make customer delete button working

MainWindow.axaml.cs

@@ -1128,4 +1128,21 @@ public partial class MainWindow : Window
                 MessageBox.Show(this, ex.StackTrace, "Fehler");
             }
     }
+
+    private void BtnDeleteCustomer_OnClick(object? sender, RoutedEventArgs e)
+    {
+        if (LstSettingsCustomers.SelectedIndex == null || LstSettingsCustomers.SelectedIndex == -1) return;
+        foreach (var customer in Settings._instance.customers.customers)
+            if (customer.ID == Settings._instance.customers.current)
+                try
+                {
+                    Settings._instance.customers.customers.Remove(customer);
+                    break;
+                }
+                catch (Exception ex)
+                {
+                    MessageBox.Show(this, "Unknown Error: " + ex.Message, "Error");
+                }
+        RefreshCustomerItems();
+    }
 }