How to get a reference for a control of the main form?

[thiforums]

Using the QT Designer I have made the following things:
1-a main form called MainFormDlg (QMainWindow) with a Tree Widget control (treeDownloads);
2-a dialog box called DownloadDlg (QDialog) containing OK and CANCEL buttons.

When the user presses the OK button in the DownloadDlg dialog, I wanna clear the content of the treeDownloads. But I don't know how to get a reference to treeDownloads which is from the main form.

My code below compiles fine, but when I press the OK button on the DownloadDlg dialog, the program simply crashes. (Please, look at my questions at the commented lines **):

//downloaddlg.cpp:

#include "downloaddlg.h"
 
DownloadDlg::DownloadDlg(QWidget *parent) :
    QDialog(parent){
    setupUi(this);
 
    connect(buttonBox, SIGNAL(accepted()), this, SLOT(addUrlsToTree()));
 
    setAttribute(Qt::WA_DeleteOnClose);
    pMainForm = (Ui_MainFormDlg*)parent;    // ** Is this correct? **
}
 
void DownloadDlg::addUrlsToTree(){
    pMainForm->treeDownloads->clear();   // ** Is this correct? **
}

#include "downloaddlg.h"

DownloadDlg::DownloadDlg(QWidget *parent) :
    QDialog(parent){
    setupUi(this);

    connect(buttonBox, SIGNAL(accepted()), this, SLOT(addUrlsToTree()));

    setAttribute(Qt::WA_DeleteOnClose);
    pMainForm = (Ui_MainFormDlg*)parent;    // ** Is this correct? **
}

void DownloadDlg::addUrlsToTree(){
    pMainForm->treeDownloads->clear();   // ** Is this correct? **
}

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//downloaddlg.h:

#ifndef DOWNLOADDLG_H
#define DOWNLOADDLG_H
 
#include <QtGui/QDialog>
#include <QtGui/QMainWindow>
#include "ui_downloaddlg.h"
#include "ui_mainform.h"
//#include "mainform.h"
 
class DownloadDlg : public QDialog, private Ui::DownloadDlg {
    Q_OBJECT
    Q_DISABLE_COPY(DownloadDlg)
public:
    explicit DownloadDlg(QWidget *parent = 0);
 
private:
    Ui_MainFormDlg *pMainForm;
 
private slots:
    void addUrlsToTree();
 
protected:
    virtual void changeEvent(QEvent *e);
};
 
#endif // DOWNLOADDLG_H

#ifndef DOWNLOADDLG_H
#define DOWNLOADDLG_H

#include <QtGui/QDialog>
#include <QtGui/QMainWindow>
#include "ui_downloaddlg.h"
#include "ui_mainform.h"
//#include "mainform.h"

class DownloadDlg : public QDialog, private Ui::DownloadDlg {
    Q_OBJECT
    Q_DISABLE_COPY(DownloadDlg)
public:
    explicit DownloadDlg(QWidget *parent = 0);

private:
    Ui_MainFormDlg *pMainForm;

private slots:
    void addUrlsToTree();

protected:
    virtual void changeEvent(QEvent *e);
};

#endif // DOWNLOADDLG_H

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