In TMS VCL UI Pack 10.5.6.1, the function TAdvStringGrid.DoFixedCellEdit in the unit AdvGrid.pas misses the var keyword before the AEditor parameter, so the editor returned by the event handler OnFixedEdit is not passed through to the caller of DoFixedCellEdit.
Thanks for informing.
We fixed this and this fix will be included in the update scheduled for this week.