Hi Jorge,
Thanks for your interest in Syncfusion products.
Query #1 "Its imposible to me to put a null value into a GridGroupingControl cell"
Your requirement to set null value for DropDownCalendarControl inside the EditItemTemplate of the Grid can be achieved by setting the following properties EnableNullDate, NullString, IsNullDate .Please refer the below code snippet.
[ASPX]
For your convenience, we have created sample and the same can be downloaded from the following link.
DateTime2111507167.zipPlease let me know if you have any other questions or concerns.
Regards,
Sridhar.N