Tryton keeps a reference to unused Combobox in editable trees
As explained in https://codereview.tryton.org/289791002/diff/313311002/tryton/tryton/gui/window/view_form/view/list_gtk/editabletree.py#newcode194 GTK will keep a reference to the editable ComboBox because the 'remove-widget' is not sent for CellRendererCombo on the focus-out.
I asked (without much success) for an explanation on GNOME's discourse: https://discourse.gnome.org/t/the-focus-out-event-on-the-cellrenderercombo-do-not-emit-the-remove-widget-signal/3173
The workaround I proposed is to launch ourselves the signal on the focus-out.