Access violation when freeing form

Question

I'm backing up a database using Devart controls. Everything runs fine until I try to free the form having the backup component(TDump) and a progress bar on it. I moved the call to the finally portion of code and checked if it was assigned before trying to free it still same problem

procedure TfrmMain.CmdBackupExecute(Sender: TObject);
var
    SaveDialog: TSaveDialog;
    QRYString: String;
    frmBackup: TfrmBackup;
    Password: String;
    BackupPassword: String;
    MasterPassword: String;
    CurrentFrame: TFrameType;
    OldUser: String;
    OldPassword: String;
begin
    dmVintage.tblSettings.Open;
    BackupPassword:= dmVintage.tblSettings.FieldByName('BackupRestorePWord').AsString;
    MasterPassword:= dmVintage.tblSettings.FieldByName('MasterPWord').AsString;
    InputPassword('Enter Backup Password', Password);
        if Password = BackupPassword then
            try
                try
                    //close current frame and change to root
                    CurrentFrame:= FrameManager.CurrentFrameType;
                    FrameManager.Clear;
                    dmVintage.connMain.LoginPrompt:= False;
                    OldUser:= dmVintage.connMain.Username;
                    OldPassword:= dmVintage.connMain.Password;
                    dmVintage.connMain.Connected:= False;
                    dmVintage.connMain.Username:= 'root';
                    dmVintage.connMain.Password:= MasterPassword;
                    dmVintage.connMain.Connect;
                    SaveDialog:= TsaveDialog.Create(frmMain);
                    SaveDialog.Filter := 'SQL file|*.sql';
                    SaveDialog.DefaultExt:= '.sql';
                    SaveDialog.FileName:= 'VintageData';
                    if SaveDialog.Execute then
                        begin
                         frmBackup:= TfrmBackup.Create(frmMain);
                         frmBackup.Show;
                         frmBackup.mdVintage.BackupToFile(AddTimestampToFilename(SaveDialog.FileName), QryString);
                         //FreeAndNil(frmBackup);
                         //FreeAndNil(SaveDialog);
                         dlgI('Backup Seccessful');
                        end;
                Except on E: Exception do
                    dlgW2('TfrmMain.CmdBackupExecute', E.Message);
                  end;
            finally
                //ShowMessage('Finally');
                if Assigned(frmBackup) then
                  FreeAndNil(frmBackup);
                if Assigned(SaveDialog) then
                  FreeAndNil(SaveDialog);

                //reset connection and load old frame
                dmVintage.connMain.Connected:= False;
                dmVintage.connMain.Username:= OldUser;
                dmVintage.connMain.Password:= OldPassword;
                dmVintage.connMain.Connect;
                dmVintage.connMain.LoginPrompt:= True;
                FrameManager.LoadFrame(CurrentFrame);
            end
        else
          dlgE('Invalid Backup Password');
end;

function TfrmMain.AddTimestampToFilename(Value: String): String;
var
  Extension: String;
  FileName: String;
  FormattedDataTime: String;
begin
  Extension:= ExtractFileExt(Value);
  FileName:= ChangeFileExt(Value, '');
  DateTimeToString(FormattedDataTime, 'yyyymmdd_hhmm', Now);
  FileName:= FileName + '_' + FormattedDataTime;
  Result:= ChangeFileExt(FileName, Extension);
end;

The Backup form is very simple with a few labels the TDump component and a progress bar.

unit uBackup;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, uDataVintage, DADump, MyDump,
  Vcl.ComCtrls, Vcl.StdCtrls;

type
  TfrmBackup = class(TForm)
    mdVintage: TMyDump;
    lblBackingUpTable: TLabel;
    lblTable: TLabel;
    Label3: TLabel;
    pbBackup: TProgressBar;
    procedure mdVintageBackupProgress(Sender: TObject; ObjectName: string;
      ObjectNum, ObjectCount, Percent: Integer);
  private
    { Private declarations }
  public
    { Public declarations }
  end;



implementation

{$R *.dfm}

procedure TfrmBackup.mdVintageBackupProgress(Sender: TObject;
  ObjectName: string; ObjectNum, ObjectCount, Percent: Integer);
begin
  Application.ProcessMessages;
  if lblTable.Caption <> ObjectName then
    lblTable.Caption:= ObjectName;
  pbBackup.Position:= Percent;
end;

end.

Answer 1

Use frmBackup.Release when freeing (especially non-modal) forms.

Freeing forms

The form is not shown in a modal way, and you couldn't, because you're controlling it from the main form, which wouldn't work on a modal form.

I think you get the access violation because you 'bluntly' free the form, while the form itself is also still visible and handling messages. Because of that, the form code (the general TForm code) might at some point still try to do something to the form, even though your instance has already been cleaned up. Even when you call 'Close` in the code, you have this issue, because closing is also not a synchronous process, and requires the form to handle messages.

In general the solution is to call frmBackup.Release instead of frmBackup.Free. That way, the form queues a message for itself. It will first handle the other stuff it has to do, and at some point encounter this message and start a graceful clean-up procedure before it finally frees itself. This is typically the way to close a form from, say, a button-click event on the form itself, but I think it will get you out of this pickle as well.

General tips on Free and FreeAndNil

You don't need to call FreeAndNil in most cases, and especially not on a local variable for which you know exactly when it was assigned a value or not. The only thing FreeAndNil does, is make your reference nil, which is not needed at all for a variable that goes out of scope three lines later anyway.

There is no need at all to call if assigned before calling FreeAndNil, or even Free. Assigned is only checking if the reference is nil, which is what Free also does internally. That right: This is valid code that won't throw errors:

var
  o: TObject;
begin
  o := nil;
  o.Free;