delphi – 为什么集中的MessageDlg会创建异常？

德尔福6.
我实现了一个以所有者表单为中心的MessageDlg
正如@David Heffernan在2011年1月6日的建议.

2011年的原始问题在这里：
How to make MessageDlg centered on owner form.

居中对话框有效一次.
在第一次抛出异常之后.
– EAccessViolation
– 地址00000000的访问冲突
– 读取地址00000000

我可能做错了什么？

function TEthernetNodes_form.CenteredMessageDlg(const Msg: string;
                                                DlgType:   TMsgDlgType;
                                                Buttons:   TMsgDlgButtons;
                                                HelpCtx:   Integer): Integer;
// Open a message Dialog in the center of the owner form
var
  Dialog: TForm;
begin
  Result := mrNo; // Suppress linker warning
  try
    Dialog := CreateMessageDialog(Msg,DlgType,Buttons);
    try
      Self.InsertComponent(Dialog);
      Dialog.Position := poOwnerFormCenter;
      Result := Dialog.ShowModal
    finally
      Dialog.Free
    end;

  except on E: Exception do
               begin
                 AddToactivitylog('Exception in CenteredMsgDlg: [' +  
                                   string(E.ClassName) + ']' +  
                                   E.Message,True,True);  
                 //Tried "ShowMEssage" instead of AddToactivitylog here. Does not display.
               end;

  end;
end;  

procedure TEthernetNodes_form.Button1Click(Sender: TObject);
begin
  CenteredMessageDlg('Test CenteredMessageDlg.',mtConfirmation,[mbOK],0);
end;

我的活动日志中显示了异常,如下所示：

Exception in CenteredMsgDlg: [EAccessViolation] Access violation at  
address 00000000. Read of address 00000000

解决方法

CreateMessageDialog使用Application作为其所有者创建表单 – 它将添加到应用程序组件列表中.使用Self.InsertComponent(Dialog);您将它添加到您的表单组件列表,但它不会从应用程序中删除.

var
  Dialog: TForm;
begin
  Result := mrNo; // Suppress linker warning
  try
    Dialog := CreateMessageDialog(Msg,Buttons);
    try
      Application.RemoveComponent(Dialog); // remove Dialog from Application components
      Self.InsertComponent(Dialog);
      Dialog.Position := poOwnerFormCenter;
      Result := Dialog.ShowModal;
    finally
      Dialog.Free
    end;