gtk_widget_problem

~~~~~~~~~~~~~~
myclass.h
~~~~~~~~~~~~~~
#include <gtk/gtk.h>
#include "DrawingArea.h"

class myclass
{
    public:
        myclass();
        virtual ~myclass();
        void newTab();

    protected:
        GtkWidget *mainWindow;
        GtkBox *box;
        GtkNotebook *View;
        GtkWidget *drawArea;
            GtkWidget *label;
        DrawingArea *drawpointer;

        bool checkFlag;
        int cPage;
};

extern "C"
{
    void on_new(GtkWidget *widget, gpointer user_data);
}


~~~~~~~~~~~~~~
myclass.cpp
~~~~~~~~~~~~~~
#include "myclass.h"
#include <iostream>

extern myclass *pointer;

myclass:: myclass()
{
GtkBuilder *builder = gtk_builder_new();
gtk_builder_add_from_file (builder, "uifile.ui", NULL);
gtk_builder_connect_signals (builder, NULL);
mainWindow = GTK_WIDGET (gtk_builder_get_object(builder, "mainWindow"));
box = GTK_BOX (gtk_builder_get_object(builder, "box1"));
gtk_widget_show_all (mainWindow);
}

myclass::~myclass()
{
}

void on_new(GtkWidget *widget, gpointer user_data)
{
pointer->newTab();
}

void myclass:: newTab()
{
if(!checkFlag)
{
    drawpointer = new DrawingArea();
    drawArea = drawpointer->createArea();
    View = GTK_NOTEBOOK(gtk_notebook_new());
    gtk_notebook_set_tab_pos (View, GTK_POS_TOP);
    label = gtk_label_new ("Document");
    gtk_notebook_append_page (View, drawArea, label);
    gtk_box_pack_start(box, GTK_WIDGET(View), TRUE, TRUE, 0);
    gtk_box_reorder_child (box, GTK_WIDGET(View), 1);
    gtk_widget_show (GTK_WIDGET(View));
    checkFlag = true;
  }
    label = gtk_label_new ("Document1");
    cPage = gtk_notebook_get_current_page (View);
    gtk_notebook_insert_page (View, drawArea, label, cPage + 1);
    gtk_notebook_next_page (GTK_NOTEBOOK (View));
}

~~~~~~~~~~~~~~
DrawingArea.h
~~~~~~~~~~~~~~
#include <gtk/gtk.h>

class DrawingArea
{
    public:
        DrawingArea();
        virtual ~DrawingArea();

        GtkWidget *drawingArea;
        GtkWidget *createArea();
};


~~~~~~~~~~~~~~
DrawingArea.cpp
~~~~~~~~~~~~~~

#include "DrawingArea.h"

DrawingArea:: DrawingArea()
{
}

DrawingArea:: ~DrawingArea()
{
}

GtkWidget* DrawingArea::createArea()
{
    drawingArea = gtk_drawing_area_new();
    gtk_widget_show (drawingArea);
    return drawingArea;
}