Thank you for contacting Syncfusion Support.
We have checked the reported crash which occurs using the RibbonControlAdv and RibbonPanelMergeContainer. We have also faced this type of designer crash in 184.108.40.206 version and it has been fixed in our next version 15.3. You can also check this issue in our latest version 220.127.116.11.
Can you please check it by upgrading to our latest version and let us know whether the issue get resolved?