I wrote a small program to test accessing a widget parent's slot. Basically, it has two classes:

Widget:

namespace Ui
{
    class Widget;
}

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent = 0);
    ~Widget();
    QLabel *newlabel;
    QString foo;

public slots:
    void changeLabel();

private:
    Ui::Widget *ui;
};

Widget::Widget(QWidget *parent)
    : QWidget(parent), ui(new Ui::Widget)
{
    ui->setupUi(this);
    customWidget *cwidget = new customWidget();
    newlabel = new QLabel("text");
    foo = "hello world";
    this->ui->formLayout->addWidget(newlabel);
    this->ui->formLayout->addWidget(cwidget);

    connect(this->ui->pushButton,SIGNAL(clicked()),cwidget,SLOT(callParentSlot()));
    connect(this->ui->pb,SIGNAL(clicked()),this,SLOT(changeLabel()));
}

void Widget::changeLabel(){
    newlabel->setText(this->foo);
}

and customWidget:

class customWidget : public QWidget
{
    Q_OBJECT
public:
    customWidget();
    QPushButton *customPB;

public slots:
    void callParentSlot();
};

customWidget::customWidget()
{
    customPB = new QPushButton("customPB");
    QHBoxLayout *hboxl = new QHBoxLayout();
    hboxl->addWidget(customPB);
    this->setLayout(hboxl);
    connect(this->customPB,SIGNAL(clicked()),this,SLOT(callParentSlot()));
}

void customWidget::callParentSlot(){
    ((Widget*)this->parentWidget())->changeLabel();
}

in the main function, I simply created an instance of Widget, and called show() on it. This Widget instance has a label, a QString, an instance of customWidget class, and two buttons (inside the ui class, pushButton and pb). One of the buttons calls a slot in its own class called changeLabel(), that, as the name suggests, changes the label to whatever is set in the QString contained in it. I made that just to check that changeLabel() worked. This button is working fine. The other button calls a slot in the customWidget instance, named callParentSlot(), that in turn tries to call the changeLabel() slot in its parent. Since in this case I know that its parent is in fact an instance of Widget, I cast the return value of parentWidget() to Widget*. This button crashes the program. I made a button within customWidget to try to call customWidget's parent slot as well, but it also crashes the program. I followed what was on this question. What am I missing?