If you use sl.Free, the object is freed but the variable sl still points to the now invalid memory.
Use FreeAndNil(sl) to both free the object and clear the pointer.
By the way, if you do:
var
sl1, sl2: TStringList;
begin
sl1 := TStringList.Create;
sl2 := sl1;
FreeAndNil(sl1);
// sl2 is still assigned and must be cleared separately (not with FreeAndNil because it points to the already freed object.)
end;
procedure TObject.Free;
asm
TEST EAX,EAX
JE @@exit // Jump to exit if pointer is nil.
MOV ECX,[EAX]
MOV DL,1
CALL dword ptr [ECX].vmtDestroy // Call cleanup code (and destructor).
@@exit:
end;