请教:Resize事件出错

我在form1的Resize事件中,设置另一个form的位置,但在窗口关闭时总提示出错,代码如下:procedure TMainForm.FormResize(Sender: TObject);
var
  rc:TRect;
begin
  rc.TopLeft := ClientToScreen(self.ClientRect.TopLeft);
  rc.Top := rc.Top + self.VideoWindow1.Top;  if(rc.Top < 0) or (rc.Left < 0) then
    exit;  if (SubTitleForm = nil) then
    exit;  SubTitleForm.Top := rc.Top;
  SubTitleForm.Left := rc.Left;
end;怎么回事,多谢指点.