Wt

When removing rows from WTreeView, it is possible that an invalid index is taken out, but triggers a segfault in

#0 Wt::WTreeViewNode::renderedRow(int, int) /home/matthias/Documents/wt/src/Wt/WTreeView.C:872                                                                                                                                               
#1 Wt::WTreeView::modelRowsAboutToBeRemoved(Wt::WModelIndex const&, int, int) /home/matthias/Documents/wt/src/Wt/WTreeView.C:2227    

Where the WTreeViewNode itself is a nullptr.
This can be the case when an invalid widget is taken. An assert exists here, which does trigger, but of course is no longer present in release binaries.

We should guard these against invalid indices:

1. a model row removal
2. WStandardItem::takeRow()