Why does the code work on an event - a button click. But it doesn't work on the event - form creation?

Question

Delphi

There is such a code.

unit UnitMain;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Data.DB, Vcl.Grids, Vcl.DBGrids,
  DBConnection, Vcl.StdCtrls;

type
  TFormMain = class(TForm)
    DBGrid1: TDBGrid;
    SelectedId: TLabel;
    Button1: TButton;
    procedure FormCreate(Sender: TObject);
    procedure GetIdFromSelectedRow(DataSet: TDataSet);
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  FormMain: TFormMain;

implementation

{$R *.dfm}

procedure TFormMain.Button1Click(Sender: TObject);
begin
  DBGrid1.DataSource.DataSet.AfterScroll := GetIdFromSelectedRow;
end;

procedure TFormMain.FormCreate(Sender: TObject);
begin
  DBGrid1.DataSource.DataSet.AfterScroll := GetIdFromSelectedRow;
end;

procedure TFormMain.GetIdFromSelectedRow(DataSet: TDataSet);
begin
  SelectedId.Caption := DataSet.FieldByName('id').AsString;
end;

end.

It gives the following error:

If you do not comment out the action when creating the form:

procedure TFormMain.FormCreate(Sender: TObject);
begin
  DBGrid1.DataSource.DataSet.AfterScroll := GetIdFromSelectedRow;
end;

But when the button is clicked, this code works and doesn't throw any error.
Why is that? I need to execute this code at application startup. How to get around this error? I tried sleep but it didn't help.

Answer 1

The form and, accordingly, the components on it have not yet been created. DBGrid1 does not yet exist when you try to access it. Try using the form's OnShow event.

Answer 2

type
  TFormMain = class(TForm)
    DBGrid1: TDBGrid;
    DBText1: TDBText;
    procedure FormCreate(Sender: TObject);
  private    { Private declarations }
  public    { Public declarations }
  end;

var
  FormMain: TFormMain;

implementation

{$R *.dfm}

procedure TFormMain.FormCreate(Sender: TObject);
begin
  DBText1.DataSource := DBGrid1.DataSource;
  DBText1.Field := DBGrid1.DataSource.DataSet.FieldByName('id')
end;

end.

Answer 3

When creating objects, pay attention to the application life cycle.
For example, your error says that the object does not exist yet. But you call its methods / properties.