qt qtablewidget qlineedit issue qdoublevalidator

#include <QWidget>
#include <QTableWidget>
#include <QHBoxLayout>
#include <QDomDocument>
#include <QFile>
#include <QFileDialog>
#include <QDoubleValidator>
#include <QLineEdit>

#include "PrintArea.h"

class MainWidget : public QWidget
{
  Q_OBJECT
public:
  explicit MainWidget(QWidget *parent = 0);

public slots:
  int saveFile();
  void newFile();
  int openFile();
  void myCellChanged(int i, int j);


signals:
//  void documentChange(QTableWidgetItem * item);

private:
  QDomElement RowToNode( QDomDocument&, const QTableWidget*, const int);
  bool xmlWrite(const QString& p);
  int xmlRead(const QString& p);

  QFileDialog* dialog;
  QHBoxLayout *horizontalLayout;
  QTableWidget *sheet;
  PrintArea *printArea;//distruggere!
  QDoubleValidator *validator;
  QLineEdit *lineEdit;

  QFile currentFile;
  bool modified;
};

//CPP file:
class Exc{};

MainWidget::MainWidget(QWidget *parent) : QWidget(parent), modified(false) {
  sheet = new QTableWidget(30,3, this);//settare righe e colonne dyn!!!!!!!!!!!!!!!!!!!!!
  horizontalLayout = new QHBoxLayout;
  printArea = new PrintArea(this);
  horizontalLayout->addWidget(sheet);
  horizontalLayout->addWidget(printArea);
  setLayout(horizontalLayout);
  connect(sheet, SIGNAL(cellChanged ( int, int)), this, SLOT(myCellChanged(int, int)));

  validator = new QDoubleValidator(this);
}


void MainWidget::myCellChanged(int i, int j)
{
  lineEdit = new QLineEdit(sheet);
  lineEdit->setValidator(validator);
  sheet->setCellWidget(i,j,lineEdit);
}