* py-interpreter-selector.c (cb_selector_changed,
cb_destroyed_interpreter): Check that tree model iterator is
valid.

* dialog-stf-export.c (cb_selection_changed,
cb_sheet_export_toggled): Check that tree model iterator is valid.

* dialog-simulation.c (update_log): Ditto.

* dialog-sheet-order.c (cb_name_edited, cb_selection_changed,
cb_toggled_lock, cb_toggled_direction, cb_toggled_visible): Ditto.

* dialog-scenarios.c (update_scenarios_treeview): Ditto.

* dialog-plugin-manager.c (cb_active_toggled): Ditto.

* dialog-doc-metadata.c (cb_dialog_doc_metadata_remove_clicked,
cb_dialog_doc_metadata_combo_prop_selected,
cb_dialog_doc_metadata_tree_prop_selected): Ditto.

* dialog-consolidate.c (adjust_source_areas,
construct_consolidate, cb_source_edited): Ditto.

* dialog-cell-sort.c (toggled, cb_toggled_descending): Ditto.