一个初级问题，在线等。

if RadioButton2.Checked then
  begin
    Edit2.Visible:=True;
    Edit3.Visible:=True;
    Memo1.Text:='Provider=SQLOLEDB.1;Password='+Edit3.Text+';Persist Security Info=True;User ID='+Edit2.Text+';Initial Catalog=mis;Data Source='+Edit1.Text;
    ChDir('D:\a');
    Memo1.Lines.SaveToFile('1.txt');
    Memo1.Lines.Free;
    AssignFile(f,'1.txt');
    Reset(f);
    Read(f,s);
    DataModule15.ADOConnection1.ConnectionString:=s;
    CloseFile(f);
    try
      DataModule15.ADOConnection1.Open;
      showmessage('连接成功！');
      Form14.Close;
    Except
    begin
      ShowMessage('数据库不存在或关闭！');
      DataModule15.ADOConnection1.ConnectionString:='Provider=SQLOLEDB.1;Integrated Security=SSPI;Persist Security Info=False;Initial Catalog=mis;Data Source='+'';
      Edit3.Text:='';
    end;
    end;
  end;
  if RadioButton1.Checked then
  begin
    Edit2.Visible:=False;
    Edit3.Visible:=False;
    Memo1.Text:= 'Provider=SQLOLEDB.1;Integrated Security=SSPI;Persist Security Info=False;Initial Catalog=mis;Data Source='+Edit1.Text;
    ChDir('D:\a');
    Memo1.Lines.SaveToFile('1.txt');
    Memo1.Lines.Free;
    AssignFile(f,'1.txt');
    Reset(f);
    Read(f,s);
    DataModule15.ADOConnection1.ConnectionString:= s;
    CloseFile(f);
    try
      DataModule15.ADOConnection1.Open;
      showmessage('连接成功！');
      Form14.Close;
    Except
    begin
      ShowMessage('数据库不存在或关闭！');
      DataModule15.ADOConnection1.ConnectionString:='Provider=SQLOLEDB.1;Integrated Security=SSPI;Persist Security Info=False;Initial Catalog=mis;Data Source='+'';
      Edit3.Text:='';
    end;
    end;
  end;这里是连接SQL数据库，两种验证模式
数据库是连接上了，只是在关闭窗体时
提示
project project1.exe raised exception class eaccessviolation with message'access violation at address 00005826.read of address 00005826'
怎么回事啊？？
谢谢了先。