MDIChild form cannot be created

Question

Have a main form (Form1) where is created a MDIForm (Form2) and a MDIChild (Form3) form respectivelly in execution time. In my tests the MDIForm (Form2) is show like expected but when try show the MDIChild (Form3) i get the following error that say:

Cannot create form. No mdi forms are currently active

Some idea about how fix this?

program Project1;

uses
  Vcl.Forms,
  Unit1 in 'Unit1.pas' {Form1},
  Unit2 in 'Unit2.pas' {Form2},
  Unit3 in 'Unit3.pas' {Form3};

{$R *.res}

begin
  Application.Initialize;
  Application.MainFormOnTaskbar := True;
  Application.CreateForm(TForm1, Form1);
  Application.Run;
end.

Form:

type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

uses
  Unit2, Unit3;

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
begin
  Form2 := TForm2.Create(Self);
  Form2.Show;
  Form3 := TForm3.Create(Form2);
  Form3.Show;
end;

end.

Answer 1

The VCL (not the Win32 API) is hard-coded to allow only the Application.MainForm to be set to fsMDIForm for hosting fsMDIChild Forms. Your MainForm is not the fsMDIForm parent Form, which is why you are getting the error.

Using a secondary Form as the fsMDIForm parent is technically possible, but not out of the box. It requires a bit of manual work hacking up the VCL's internals to make it work, and even then there are holes and gotchas. See my Multiple MDI Parent Forms in a single Application submission on CodeCentral for an example (I haven't updated it in over a decade, so it may need some tweaking for modern VCL versions). The old Quality Central (not Quality Portal!) ticket it refers to can be found on archive.org: #12006: Hosting MDI child forms in non-MainForm forms.

That being said, MDI is a dead technology, Microsoft abandoned it a long time ago, and modern Windows versions have poor support for MDI, especially when Visual Styles are used. You are best off not even bothering with MDI in modern software, there are other/better UI design choices available.