I have a Segmentation-fault problem in my program but i don't understand the why(perhaps is a Qt bug!!)

This is my debug:
Qt Code: 
  1. Program received signal SIGSEGV, Segmentation fault.
  2. [Switching to Thread -1226594624 (LWP 7388)]
  3. QGraphicsItem::setPos (this=0xffece9d8, pos=@0xbf9c2f60)
  4.     at graphicsview/qgraphicsitem.cpp:2053
  5. 2053    graphicsview/qgraphicsitem.cpp: No such file or directory.
  6.         in graphicsview/qgraphicsitem.cpp
  7. (gdb) bt
  8. #0  QGraphicsItem::setPos (this=0xffece9d8, pos=@0xbf9c2f60)
  9.     at graphicsview/qgraphicsitem.cpp:2053
  10. #1  0x08096f11 in MarginDialog::on_spinBox_3_valueChanged ()
  11. #2  0x080a3dfb in MarginDialog::qt_metacall ()
  12. #3  0xb7312131 in QMetaObject::activate (sender=0x8294fb8,
  13.     from_signal_index=<value optimized out>, to_signal_index=34,
  14.     argv=<value optimized out>) at kernel/qobject.cpp:3001
  15. #4  0xb73127c2 in QMetaObject::activate (sender=0x8294fb8, m=0xb7f2d04c,
  16.     local_signal_index=0, argv=0xbf9c305c) at kernel/qobject.cpp:3071
  17. #5  0xb7d2c623 in QSpinBox::valueChanged (this=0x8294fb8, _t1=10)
  18.     at .moc/release-shared/moc_qspinbox.cpp:132
  19. #6  0xb7b36519 in QSpinBoxPrivate::emitSignals (this=0x8295088,
  20.     ep=EmitIfChanged, old=@0xbf9c30e8) at widgets/qspinbox.cpp:951
  21. #7  0xb7a8aa0c in QAbstractSpinBoxPrivate::setValue (this=0x8295088,
  22.     val=@0xbf9c3140, ep=EmitIfChanged, doUpdate=true)
  23.     at widgets/qabstractspinbox.cpp:1645
  24. #8  0xb7b34b4e in QSpinBox::setValue (this=0x8294fb8, value=10)
  25.     at widgets/qspinbox.cpp:255
  26. #9  0x08096e9e in MarginDialog::resetSpinBox ()
  27. #10 0x08076f7a in XmlGenerator::parseFile ()
  28. #11 0x0805ecf7 in MainWindow::on_actionOpen_triggered ()
  29. #12 0x080a2f5d in MainWindow::qt_metacall ()
  30. #13 0xb7312131 in QMetaObject::activate (sender=0x813c3b0,
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I don't see any 0x0 value.............................