Allow assigning null to Window.Icon.

src/Avalonia.Controls/Window.cs

@@ -129,7 +129,7 @@ namespace Avalonia.Controls
 
             ShowInTaskbarProperty.Changed.AddClassHandler<Window>((w, e) => w.PlatformImpl?.ShowTaskbarIcon((bool)e.NewValue));
 
-            IconProperty.Changed.AddClassHandler<Window>((s, e) => s.PlatformImpl?.SetIcon(((WindowIcon)e.NewValue).PlatformImpl));
+            IconProperty.Changed.AddClassHandler<Window>((s, e) => s.PlatformImpl?.SetIcon(((WindowIcon)e.NewValue)?.PlatformImpl));
 
             CanResizeProperty.Changed.AddClassHandler<Window>((w, e) => w.PlatformImpl?.CanResize((bool)e.NewValue));
 

src/Avalonia.X11/X11Window.cs

@@ -995,11 +995,11 @@ namespace Avalonia.X11
 
         public void SetIcon(IWindowIconImpl icon)
         {
-            var data = ((X11IconData)icon).Data;
+            var data = ((X11IconData)icon)?.Data;
             fixed (void* pdata = data)
                 XChangeProperty(_x11.Display, _handle, _x11.Atoms._NET_WM_ICON,
                     new IntPtr((int)Atom.XA_CARDINAL), 32, PropertyMode.Replace,
-                    pdata, data.Length);
+                    pdata, data?.Length ?? 0);
         }
 
         public void ShowTaskbarIcon(bool value)

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -942,7 +942,7 @@ namespace Avalonia.Win32
         public void SetIcon(IWindowIconImpl icon)
         {
             var impl = (IconImpl)icon;
-            var hIcon = impl.HIcon;
+            var hIcon = impl?.HIcon ?? IntPtr.Zero;
             UnmanagedMethods.PostMessage(_hwnd, (int)UnmanagedMethods.WindowsMessage.WM_SETICON,
                 new IntPtr((int)UnmanagedMethods.Icons.ICON_BIG), hIcon);
         }