Добавьте файлы проекта.

zooManagerPro/App.axaml.cs

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+using Avalonia;
+using Avalonia.Controls.ApplicationLifetimes;
+using Avalonia.Data.Core;
+using Avalonia.Data.Core.Plugins;
+using Avalonia.Markup.Xaml;
+using System.Linq;
+using zooManagerPro.ViewModels;
+using zooManagerPro.Views;
+
+namespace zooManagerPro
+{
+    public partial class App : Application
+    {
+        public override void Initialize()
+        {
+            AvaloniaXamlLoader.Load(this);
+        }
+
+        public override void OnFrameworkInitializationCompleted()
+        {
+            if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
+            {
+                desktop.MainWindow = new MainWindow
+                {
+                    DataContext = new MainWindowViewModel(),
+                };
+            }
+
+            base.OnFrameworkInitializationCompleted();
+        }
+    }
+}