Hi Waruna,
Thanks for your interest in Syncfusion Products.
Please set the EnableDocking to false before closing the form.
this.dockingManager1.SetEnableDocking(form, false);
form.Close();
Please refer to the attached sample and let me know if you need further assistance with this incident.
http://websamples.syncfusion.com/samples/Tools.Windows/F59599/main.htmBest Regards,
Poly