A drop handler with the "child" drop type (i.e., dropping onto a row rather than in a specific order between rows) receives the wrong row object (the one row below). In particular, in a tree grid, dropping onto the last row of a subtree always returns null as the row object.

Test

In tl-demo, in the "Technical Demo" view : Components: Drag & Drop, drop work packages onto the appropriate assignees in the tree grid.