Fix segfault for Ctrl+C in neptune 3 ui

process ./neptune3-ui (14574) crashed 

why: uncaught signal 11 (Segmentation fault)

where: main thread, TID: 14574, pthread ID: 0x7f09b6ed5800

backtrace:
   1: ./neptune3-ui(+0x18c564) [0x5575ad599564]
   2: ./neptune3-ui(+0x18c7f3) [0x5575ad5997f3]
   3: ./neptune3-ui(+0x18adb0) [0x5575ad597db0]
   4: /lib/x86_64-linux-gnu/libpthread.so.0(+0x12890) [0x7f09b2f0e890]
   5: QList<CursorNavigationAttached>::removeOne(CursorNavigationAttached const&) [+0x40]
   6: CursorNavigation::unregisterItem(CursorNavigationAttached) [+0x26]
   7: CursorNavigationAttached::onWindowChanged(QQuickWindow) [+0x98]
   8: /opt/qt/auto/5.14.2/lib/libQt5Core.so.5(+0x2dd2b7) [0x7f09b360b2b7]
   9: QQuickItem::windowChanged(QQuickWindow*) [+0x42]
  10: QQuickItemPrivate::itemChange(QQuickItem::ItemChange, QQuickItem::ItemChangeData const&) [+0x235]
  11: QQuickItemPrivate::derefWindow() [+0x265]
  12: QQuickItemPrivate::derefWindow() [+0x208]
  13: QQuickItemPrivate::derefWindow() [+0x208]
  14: QQuickItemPrivate::derefWindow() [+0x208]
  15: QQuickItemPrivate::derefWindow() [+0x208]
  16: QQuickItemPrivate::derefWindow() [+0x208]
  17: QQuickItemPrivate::derefWindow() [+0x208]
  18: QQuickItemPrivate::derefWindow() [+0x208]
  19: QQuickItemPrivate::derefWindow() [+0x208]
  20: QQuickItemPrivate::derefWindow() [+0x208]
  21: QQuickShaderEffectSource::itemChange(QQuickItem::ItemChange, QQuickItem::ItemChangeData const&) [+0x5d]
  22: QQuickItemPrivate::itemChange(QQuickItem::ItemChange, QQuickItem::ItemChangeData const&) [+0x235]