Form destroy problem after D5->D6


I discovered the same problem with my form containing TStringGrid control among others, while migrating from Delphi 4 to 6.
I could track the destroy process and see that all these  controls inside this form have got not 'nil' FHandle and the statement:
if FHandle <> 0 then DestroyWindowHandle;
in the
destructor TWinControl.Destroy;
of the 'Controls' class, Controls.pas fails.

I've found no explanation!