Fix a possible cause of 'System.ObjectDisposedException'

src/PixiEditor/Models/Controllers/MouseUpdateController.cs

@@ -42,7 +42,7 @@ public class MouseUpdateController : IDisposable
 
     public void Dispose()
     {
-        _timer.Dispose();
         element.MouseMove -= OnMouseMove;
+        _timer.Dispose();
     }
 }