handle platforms that return null for the trayiconimpl.

src/Avalonia.Controls/Platform/IWindowingPlatform.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 namespace Avalonia.Platform
 {
     public interface IWindowingPlatform
@@ -6,6 +8,6 @@ namespace Avalonia.Platform
 
         IWindowImpl CreateEmbeddableWindow();
 
-        ITrayIconImpl CreateTrayIcon();
+        ITrayIconImpl? CreateTrayIcon();
     }
 }

src/Avalonia.Controls/Platform/PlatformManager.cs

@@ -1,8 +1,9 @@
 using System;
 using System.Reactive.Disposables;
-using Avalonia.Media;
 using Avalonia.Platform;
 
+#nullable enable
+
 namespace Avalonia.Controls.Platform
 {
     public static partial class PlatformManager
@@ -22,7 +23,7 @@ namespace Avalonia.Controls.Platform
         {
         }
 
-        public static ITrayIconImpl CreateTrayIcon ()
+        public static ITrayIconImpl? CreateTrayIcon ()
         {
             var platform = AvaloniaLocator.Current.GetService<IWindowingPlatform>();
 

src/Avalonia.Controls/TrayIcon.cs

@@ -16,15 +16,18 @@ namespace Avalonia.Controls
 
     public class TrayIcon : AvaloniaObject, INativeMenuExporterProvider, IDisposable
     {
-        private readonly ITrayIconImpl _impl;
+        private readonly ITrayIconImpl? _impl;
 
-        private TrayIcon(ITrayIconImpl impl)
+        private TrayIcon(ITrayIconImpl? impl)
         {
-            _impl = impl;
+            if (impl != null)
+            {
+                _impl = impl;
 
-            _impl.SetIsVisible(IsVisible);
+                _impl.SetIsVisible(IsVisible);
 
-            _impl.OnClicked = () => Clicked?.Invoke(this, EventArgs.Empty);
+                _impl.OnClicked = () => Clicked?.Invoke(this, EventArgs.Empty);
+            }
         }
 
         public TrayIcon () : this(PlatformManager.CreateTrayIcon())
@@ -160,25 +163,25 @@ namespace Avalonia.Controls
 
             if(change.Property == IconProperty)
             {
-                _impl.SetIcon(Icon.PlatformImpl);
+                _impl?.SetIcon(Icon.PlatformImpl);
             }
             else if (change.Property == IsVisibleProperty)
             {
-                _impl.SetIsVisible(change.NewValue.GetValueOrDefault<bool>());
+                _impl?.SetIsVisible(change.NewValue.GetValueOrDefault<bool>());
             }
             else if (change.Property == ToolTipTextProperty)
             {
-                _impl.SetToolTipText(change.NewValue.GetValueOrDefault<string?>());
+                _impl?.SetToolTipText(change.NewValue.GetValueOrDefault<string?>());
             }
             else if (change.Property == MenuProperty)
             {
-                _impl.MenuExporter?.SetNativeMenu(change.NewValue.GetValueOrDefault<NativeMenu>());
+                _impl?.MenuExporter?.SetNativeMenu(change.NewValue.GetValueOrDefault<NativeMenu>());
             }
         }
 
         /// <summary>
         /// Disposes the tray icon (removing it from the tray area).
         /// </summary>
-        public void Dispose() => _impl.Dispose();
+        public void Dispose() => _impl?.Dispose();
     }
 }

src/Avalonia.DesignerSupport/Remote/PreviewerWindowingPlatform.cs

@@ -17,7 +17,7 @@ namespace Avalonia.DesignerSupport.Remote
         private static PreviewerWindowImpl s_lastWindow;
         public static List<object> PreFlightMessages = new List<object>();
 
-        public ITrayIconImpl CreateTrayIcon() => new TrayIconStub();
+        public ITrayIconImpl CreateTrayIcon() => null;
 
         public IWindowImpl CreateWindow() => new WindowStub();
 

src/Avalonia.DesignerSupport/Remote/TrayIconStub.cs

@@ -1,40 +0,0 @@
-using System;
-using Avalonia.Controls;
-using Avalonia.Controls.Platform;
-using Avalonia.Platform;
-
-namespace Avalonia.DesignerSupport.Remote
-{
-    class TrayIconStub : ITrayIconImpl
-    {
-        public Action Clicked { get; set; }
-        public Action DoubleClicked { get; set; }
-        public Action RightClicked { get; set; }
-
-        public INativeMenuExporter MenuExporter => null;
-
-        public Action OnClicked { get; set; }
-
-        public void Dispose()
-        {
-            throw new NotImplementedException();
-        }
-
-        public void SetIcon(IWindowIconImpl icon)
-        {   
-        }
-
-        public void SetIsVisible(bool visible)
-        {   
-        }
-
-        public void SetMenu(NativeMenu menu)
-        {
-            throw new NotImplementedException();
-        }
-
-        public void SetToolTipText(string text)
-        {
-        }
-    }
-}

src/Avalonia.Headless/AvaloniaHeadlessPlatform.cs

@@ -52,10 +52,7 @@ namespace Avalonia.Headless
 
             public IPopupImpl CreatePopup() => new HeadlessWindowImpl(true);
 
-            public ITrayIconImpl CreateTrayIcon()
-            {
-                throw new NotImplementedException();
-            }
+            public ITrayIconImpl CreateTrayIcon() => return null;
         }
         
         internal static void Initialize()

src/iOS/Avalonia.iOS/Stubs.cs

@@ -22,7 +22,7 @@ namespace Avalonia.iOS
 
         public IWindowImpl CreateEmbeddableWindow() => throw new NotSupportedException();
 
-        public ITrayIconImpl CreateTrayIcon() => throw new NotSupportedException();
+        public ITrayIconImpl CreateTrayIcon() => null;
     }
     
     class PlatformIconLoaderStub : IPlatformIconLoader

tests/Avalonia.Controls.UnitTests/WindowingPlatformMock.cs

@@ -27,7 +27,7 @@ namespace Avalonia.Controls.UnitTests
 
         public ITrayIconImpl CreateTrayIcon()
         {
-            throw new NotImplementedException();
+            return null;
         }
 
         public IPopupImpl CreatePopup() => _popupImpl?.Invoke() ?? Mock.Of<IPopupImpl>(x => x.RenderScaling == 1);

tests/Avalonia.UnitTests/MockWindowingPlatform.cs

@@ -128,7 +128,7 @@ namespace Avalonia.UnitTests
 
         public ITrayIconImpl CreateTrayIcon()
         {
-            throw new NotImplementedException();
+            return null;
         }
 
         private static void SetupToplevel<T>(Mock<T> mock) where T : class, ITopLevelImpl