Hi Wenkai,
Thanks for contacting Syncfusion Support.
We have checked the reported query “if the drop target sfdatagrid has no item (for example UserDetails1 is empty in that demo), it is not possible to drop an item into” from our side. we are able to reproduce the reported scenario. We have forwarded it to development team. We will update you with the details in two business days (February 25, 2019).
Regards,
Jayaleshwari N.