OutlineModel: Potential fix for crashes-on-exit

Valgrind trace:

9586== at 0x40F583B:

KDevelop::DocumentController::activateDocument(KDevelop::IDocument*,
KTextEditor::Range const&) (documentcontroller.cpp:793)

9586== by 0x2B4E3CBC: OutlineModel::activate(QModelIndex const&)

(outlinemodel.cpp:215)

9586== by 0x2B4E489E: OutlineWidget::activated(QModelIndex)

(outlinewidget.cpp:93)

9586== by 0x2B4E4AFA: call (qobjectdefs_impl.h:500)

9586== by 0x2B4E4AFA: call<QtPrivate::List<const QModelIndex &>,

void> (qobjectdefs_impl.h:519)

9586== by 0x2B4E4AFA: QtPrivate::QSlotObject<void

(OutlineWidget::*)(QModelIndex), QtPrivate::List<QModelIndex const&>,
void>::impl(int, QtPrivate::QSlotObjectBase*, QObject*, void**, bool*)
(qobject_impl.h:143)

9586== by 0x9F78616: QMetaObject::activate(QObject*, int, int,

void**) (in /usr/lib/x86_64-linux-gnu/libQt5Core.so.5.4.2)

9586== by 0x9362DC4: QAbstractItemView::activated(QModelIndex

const&) (in /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5.4.2)

9586== by 0x9366E4D:

QAbstractItemView::mouseReleaseEvent(QMouseEvent*) (in
/usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5.4.2)

9586== by 0x93B0B92: QTreeView::mouseReleaseEvent(QMouseEvent*) (in

/usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5.4.2)

9586== by 0x9154187: QWidget::event(QEvent*) (in

/usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5.4.2)