add nullable to type instead of assigning null to make rider happy :)

src/Avalonia.X11/X11PlatformLifetimeEvents.cs

@@ -241,6 +241,6 @@ namespace Avalonia.X11
                 IntPtr.Zero);
         }
 
-        public event EventHandler<ShutdownRequestedEventArgs> ShutdownRequested = null!;
+        public event EventHandler<ShutdownRequestedEventArgs>? ShutdownRequested;
     }
 }