Segmentation fault error

[Niamita]

Hi all

I am facing a error segmentation fault in the following code

mapper = new QSignalMapper ();
 
    for(int i=0; i<obj_schedule_table->rowCount(); i++)
    {
        QWidget *widget1;
        widget1 = obj_schedule_table->cellWidget(i,3);
        QComboBox *combo = dynamic_cast<QComboBox*>(widget1->children().at(1));
        combo->setEditable(false);
 
        mapper->setMapping(combo, i);
        connect(combo, SIGNAL(activated(QString)), mapper, SLOT (map()));
    }
    connect(mapper, SIGNAL(mapped(int)), this, SLOT(set_item_combobox_type(int)));

mapper = new QSignalMapper ();

    for(int i=0; i<obj_schedule_table->rowCount(); i++)
    {
        QWidget *widget1;
        widget1 = obj_schedule_table->cellWidget(i,3);
        QComboBox *combo = dynamic_cast<QComboBox*>(widget1->children().at(1));
        combo->setEditable(false);

        mapper->setMapping(combo, i);
        connect(combo, SIGNAL(activated(QString)), mapper, SLOT (map()));
    }
    connect(mapper, SIGNAL(mapped(int)), this, SLOT(set_item_combobox_type(int)));

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I know it may be due to my code is pointing some null pointer but i am not understanding what is the cause of error.

Thanks in advance.