Hi Elsa,
You can add a plus sign button and show a drop down grid with the error list at a particular row through code. This can be achieved by using a custom cell type. I have attached a sample in which, the GridListControl is used as the drop down grid to show the error list. Let us know if you are looking for something different. Also take a look at the VirtTreeGrid sample in \Syncfusion\Essential Studio\4.1.0.51\windows\Grid.Windows\Samples\In Depth\VirtTreeGrid\
Regards,
Calvin.
40554.zip