I have VCL application in delphi. One main form and many child forms.
How to ensure only one form opened at a time. In other words, If a form is opened, it will close previous form. Or, if user try to open form that same with previous, it will do nothing.
Code to open form in my main form:

```
procedure TFMainForm.OpenForm(const classname: string);
var
  c: TPersistentClass;
  f: TForm;
begin
    c := GetClass(classname);
    if c <> nil then
    begin
      f := TForm(TControlClass(c).Create(nil));
      f.Parent := Self;
      f.Show;
    end;
end;

```

The child form is self-freed on close event.