Allow multiple MDI Parent Forms on same Application - delphi

Question

I'm trying follow what was suggested in this answer, changing this part of Vcl.Forms.pas:
procedure TCustomForm.CreateWindowHandle(const Params: TCreateParams);
var
CreateStruct: TMDICreateStruct;
NewParams: TCreateParams;
begin
if (FormStyle = fsMDIChild) and not (csDesigning in ComponentState) then
begin
{if (Application.MainForm = nil) or
(Application.MainForm.ClientHandle = 0) then
raise EInvalidOperation.Create(SNoMDIForm);}
with CreateStruct do
begin
szClass := Params.WinClassName;
szTitle := Params.Caption;
hOwner := THandle(HInstance);
X := Params.X;
Y := Params.Y;
cX := Params.Width;
cY := Params.Height;
style := Params.Style;
lParam := THandle(Params.Param);
end;
WindowHandle := SendStructMessage(Application.MainForm.ClientHandle,
WM_MDICREATE, 0, CreateStruct);
Include(FFormState, fsCreatedMDIChild);
end
else
//...
but still comes the error saying that "no MDI Form is active"
What more is need be made to this suggestion works? Thanks in advance.
Code of test with Forms:
procedure TForm1.Button1Click(Sender: TObject);
begin
Form2 := TForm2.Create(Self); // MDIForm
Form2.Show;
Form3 := TForm3.Create(Form2); // MDIChild
Form3.Show;
end;

Answer 1

After the help of comments above (mainly of #Remy Lebeau) follows this code working. I hope that can help someone ahead :-).
// MainForm
type
TForm1 = class(TForm)
Button1: TButton;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
var
Form1: TForm1;
implementation
uses
Unit2;
{$R *.dfm}
procedure TForm1.Button1Click(Sender: TObject);
begin
Form2 := TForm2.Create(Application);
Form2.Show;
end;
// MDIForm
type
TForm2 = class(TForm)
MainMenu1: TMainMenu;
O1: TMenuItem;
procedure O1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
var
Form2: TForm2;
implementation
uses
Unit3;
{$R *.dfm}
procedure TForm2.O1Click(Sender: TObject);
begin
Form3 := TForm3.Create(Self);
Form3.Show;
end;
// MDIChild
type
TForm3 = class(TForm)
procedure FormClose(Sender: TObject; var Action: TCloseAction);
private
{ Private declarations }
procedure CreateWindowHandle(const Params: TCreateParams); override;
procedure DestroyWindowHandle; override;
protected
FMDIClientHandle: HWND;
public
{ Public declarations }
end;
var
Form3: TForm3;
implementation
uses
Unit1;
{$R *.dfm}
procedure TForm3.CreateWindowHandle(const Params: TCreateParams);
var
CreateStruct: TMDICreateStruct;
function GetMDIClientHandle: HWND;
begin
Result := 0;
if (Owner is TForm) then
Result := TForm(Owner).ClientHandle;
if (Result = 0) and (Application.MainForm <> nil) then
Result := Application.MainForm.ClientHandle;
if Result = 0 then
raise EInvalidOperation.Create('No Parent MDI Form');
end;
begin
if (FormStyle = fsMDIChild) and not (csDesigning in ComponentState) then
begin
FMDIClientHandle := GetMDIClientHandle;
with CreateStruct do
begin
szClass := Params.WinClassName;
szTitle := Params.Caption;
hOwner := HInstance;
X := Params.X;
Y := Params.Y;
cX := Params.Width;
cY := Params.Height;
style := Params.Style;
lParam := Longint(Params.Param);
end;
WindowHandle := SendMessage(FMDIClientHandle, WM_MDICREATE, 0, LongInt(#CreateStruct));
Include(FFormState, fsCreatedMDIChild);
end
else
begin
FMDIClientHandle := 0;
inherited CreateWindowHandle(Params);
Exclude(FFormState, fsCreatedMDIChild);
end;
end;
procedure TForm3.DestroyWindowHandle;
begin
if fsCreatedMDIChild in FFormState then
SendMessage(FMDIClientHandle, WM_MDIDESTROY, Handle, 0)
else
inherited DestroyWindowHandle;
FMDIClientHandle := 0;
end;
procedure TForm3.FormClose(Sender: TObject; var Action: TCloseAction);
begin
Action := CaFree;
end;