some fixes

src/Avalonia.FreeDesktop/DBusHelper.cs

@@ -12,7 +12,7 @@ namespace Avalonia.FreeDesktop
         /// This class uses synchronous execution at DBus connection establishment stage
         /// then switches to using AvaloniaSynchronizationContext
         /// </summary>
-        class DBusSyncContext : SynchronizationContext
+        private class DBusSyncContext : SynchronizationContext
         {
             private SynchronizationContext _ctx;
             private object _lock = new object();

src/Avalonia.FreeDesktop/DBusTrayIconImpl.cs

@@ -138,7 +138,7 @@ namespace Avalonia.FreeDesktop
 
         private void DestroyTrayIcon()
         {
-            if (_connection is null || !_serviceConnected || _isDisposed)
+            if (_connection is null || !_serviceConnected || _isDisposed || _statusNotifierItemDbusObj is null)
                 return;
 
             _connection.UnregisterObject(_statusNotifierItemDbusObj);

src/Avalonia.X11/X11TrayIconImpl.cs

@@ -15,7 +15,6 @@ namespace Avalonia.X11
         }
 
         private readonly DBusTrayIconImpl _dBusTrayIcon;
-
         private readonly XEmbedTrayIconImpl _xEmbedTrayIcon;
         private bool _isDisposed;