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TfrmPerson = class(TForm)
  ...
  procedure btnFilterClick(Sender: TObject);
private
  FFilterForm: TForm;
  ...
end;

{ ---------------------------------------------------------------------------- }
procedure TfrmPerson.btnFilterClick(Sender: TObject);
begin
  Screen.Cursor:= crHourGlass;
  try
    if not Assigned(FFilterForm) then
    begin
      FFilterForm:= TfrmPersonFilter.Create(Application);
      TfrmPersonFilter(FFilterForm).Init(self);
    end;
    FFilterForm.ShowModal;
  finally
    Screen.Cursor:= crDefault;
  end;
end;

{ ---------------------------------------------------------------------------- }
destructor TfrmPerson.Destroy;
begin
  if not(csDestroying in Application.ComponentState) then
  begin
    FFilterForm.Free;
    FFilterForm:= nil;
  end;
  inherited Destroy;
end;