Hi Mineev,
I am afraid that I am not able to reproduce the issue of red cross being displayed on clicking the left frame of the customize window after making several clicks on the right frame with the attached sample.
Could you please modify the attached sample so that the issue could be reproduced? This will help us to resolve the issue soon.
Thank you for your interest in Syncfusion products.
http://websamples.syncfusion.com/samples/Tools.Windows/F60921/Main.htmBest regards,
Golda