unit srcActiveFlor;
  ...
type
    TActiveFlor = class(TForm)
    procedure UpdateTables(Sender: TObject);
    procedure Button1Click(sender: Tobject);
    ...
var
ActiveFlor: TActiveFlor;
implementation
{ TActiveFlor }

procedure TActiveFlor.Button1Click(Sender: TObject);
var 
CheckForm: TcheckForm;
begin
   CheckForm:=TCheckForm.create(ActiveFlor);
   CheckForm.ShowModal;
   CheckForm.Free;
end;

procedure TActiveFlor.UpdateTables(Sender: TObject);
begin
 Label1.Caption:=inttostr(ActiveFlor.ComponentCount);//ошибка выдаеться access violation. (ActiveFlor = nil)
 ...                                               //componentcount не знает с какой формы надо считать. Почему?
end;

end.

unit srcCheckForm;
interface
uses
  ...
type
    TCheckForm = class(TForm)
    procedure Button1Click(sender: Tobject);
    ...
var
CheckForm:TCheckForm;
implementation
{ CheckFlor }

procedure CheckForm.Button1Click(Sender: TObject);
begin
   ActiveFlor.UpdateTables(Sender);
   ModalResult:=mrOk;   
end;

end.