Premise: The following changes are only after-the-fact crash prevention.

The root question of why the ERD is NULL is unclear.

(In reply to Gabriela Serventi from comment #5)
> [...]
> But, the first ERD is completely unusable. For example, if I press right
> mouse button -> Export as Picture on the ERD, Umbrello crash again with this
> backtrace:
> [...]
> Thread 1 (Thread 0xb77387c0 (LWP 2558)):
> [KCrash Handler]
> #6  0x4bc134c4 in QGraphicsView::scene() const () from /lib/libQtGui.so.4
> #7  0x08333d72 in UMLListView::slotMenuSelection(QAction*) ()
> #8  0x08334c10 in UMLListView::contextMenuEvent(QContextMenuEvent*) ()

umbrello/umllistview.cpp function slotMenuSelection()
- In case ListPopupMenu::mt_Export_Image, guard against NULL return value
  from m_doc->findView(), and guard against its umlScene() being NULL.

> [...]
> And If I press right mouse button ->  Duplicate on the ERD, Umbrello crash
> again with this backtrace:
> [...]
> Thread 1 (Thread 0xb76fd7c0 (LWP 3188)):
> [KCrash Handler]
> #6  0x4bc134c4 in QGraphicsView::scene() const () from /lib/libQtGui.so.4
> #7  0x080e99cd in UMLClipboard::copy(bool) ()
> #8  0x08313625 in UMLApp::editCutCopy(bool) ()
> #9  0x08313869 in UMLApp::slotEditCopy() ()
> #10 0x08333b7d in UMLListView::slotMenuSelection(QAction*) ()
> #11 0x08334c10 in UMLListView::contextMenuEvent(QContextMenuEvent*) ()
> #12 0x4b58c337 in QWidget::event(QEvent*) () from /lib/libQtGui.so.4

umbrello/clipboard/umlclipboard.cpp function copy()
- In case fromView==true, guard against UMLApp::app()->currentView() being
  NULL, and also guard against its umlScene() being NULL.

CCBUG: 338983