Criticals printed on console for each thumbnail when in tree view mode
Evince prints one of this:
(evince:15035): Gtk-CRITICAL **: 14:20:06.863: gtk_widget_queue_draw: assertion 'GTK_IS_WIDGET (widget)' failed
for each thumbnail being rendered when in tree view mode. This was a slip over from my previous commit 6480c703 .
I'm pushing a MR to fix it.