'Application.MainPage.set' is obsolete: 'This property is deprecated. Initialize your application by overriding Application.CreateWindow rather than setting MainPage. To modify the root page in an active application, use Windows[0].Page for applications with a single window. For applications with multiple windows, use Application.Windows to identify and update the root page on the correct window. Additionally, each element features a Window property, accessible when it's part of the current window.'