Why SIGSEGV Exception here

Question

I am trying following code to create a simple GUI application:

program RnTFormclass;
{$mode objfpc}
uses
    {$IFDEF UNIX}{$IFDEF UseCThreads}
      cthreads,
    {$ENDIF}{$ENDIF}
    Interfaces, Forms, StdCtrls;

type
    RnTForm = class(TForm)
    private
        wnd: TForm;
        btn: TButton;
    public
        constructor create;
        procedure showit; 
    end; 

constructor RnTForm.create;
    begin
        Application.Initialize;
        wnd := TForm.Create(Application);
        with wnd do begin
            Height  := 300;
            Width   := 400;
            Position:= poDesktopCenter;
            Caption := 'LAZARUS WND';
            end;
        btn := TButton.Create(wnd);
            with btn do begin
            SetBounds(0, 0, 100, 50);
            Caption := 'Click Me';
            Parent  := wnd;
            end;
    end;

procedure RnTForm.showit;
    begin
        wnd.ShowModal; {Error at this line: Throws exception External: SIGSEGV }
    end;

var 
    myform1: RnTForm; 
begin
    myform1.create; 
    myform1.showit; 
end.

However, it is throwing exception as mentioned as a comment in code above. Where is the problem and how can it be solved?

Answer 1

myform1.Create should be myform1 := RnTForm.Create.

In your code above, myform1 is a nil pointer (since it is a global variable, it is initialized to nil) until you assign something to it, in this case a (pointer to a) new instance of a RnTForm.

And of course, if myform1 is a nil pointer, you cannot use it as if it was indeed pointing to an object (so myform1.showit will not work).