小心使用可修改的常量。

procedure TFDBForm_EditForm_MD.EnableDataSetEvent(AEnalbed: Boolean);
{$J+}
const
  CDetailBeforeEditEvent: TDataSetNotifyEvent = nil;
  CDetailBeforeDeleteEvent: TDataSetNotifyEvent = nil;
{$J-}
begin
  inherited;
  if AEnalbed then
  begin
    if not Assigned(CDetailBeforeEditEvent) then CDetailBeforeEditEvent := OnDetailDataSetBeforeEdit;//
    if not Assigned(CDetailBeforeDeleteEvent) then CDetailBeforeDeleteEvent := OnDetailDataSetBeforeDelete;//
    DetailDataSet.BeforeEdit := CDetailBeforeEditEvent;
    DetailDataSet.BeforeDelete := CDetailBeforeDeleteEvent;
    DetailDataSet.BeforeInsert := OnDetailDataSetBeforeInsert;
  end else
  begin
    DetailDataSet.BeforeEdit := nil;
    DetailDataSet.BeforeDelete := nil;
    DetailDataSet.BeforeInsert := nil;
  end;
end;

上述代码似乎没有什么问题。
但是在窗体关闭再打开之后就出现问题了
    if not Assigned(CDetailBeforeEditEvent) then CDetailBeforeEditEvent := OnDetailDataSetBeforeEdit;//
    if not Assigned(CDetailBeforeDeleteEvent) then CDetailBeforeDeleteEvent := OnDetailDataSetBeforeDelete;//
注意这两句，在第一次被赋值以后，即使窗体关闭(Free)，再次打开之后,因为CDetailBeforeEditEvent 是常量，还记录了前次的地址，但前次的地址中所指的事件已经销毁了，所以会引发错误。