text: #include <nana/gui.hpp>
  #include <nana/gui/place.hpp>
  #include <nana/gui/widgets/menubar.hpp>
  
  int main(int argc, char **argv)
  {
      nana::form fm;
      nana::place pl(fm);
      nana::menubar mn(fm);
  
      pl.div("<vertical <menu weight=25>>");
      pl.field("menu") << mn; 
  
      mn.push_back("test");
      mn.at(0).append("open modal window", [&fm](nana::menu::item_proxy& ip)
      {
          nana::form fm2(fm);
          fm2.show();
          fm2.modality();
      });
  
      pl.collocate();
  
      fm.show();
  
      nana::exec();
  
      return 0;
  }
  
   // NOW FIXED !!
      //After opening fm2 via the Menu and closing the Form, the error occurs.
  
      //last callstack entry is:
      //0 0x00000000005781b6 std::unique_ptr<nana::basic_event<nana::arg_mouse>::docker, std::default_delete<nana::basic_event<nana::arg_mouse>::docker> >::~unique_ptr
  
      //if i understand correctly, the error occurs in the destructor of an unique_ptr.
  
      //The system i'm using is nana with the latest master branch, windows 8.1 with mingw (gcc 4.9.2).
  
      //If you need further information please let me know.
  
      //best regards, jan
      //^
  
      //additional note:
      //if you open a window via a button or something else, no error occurs. this makes me guess the error is relating to menubars
  