Export : set format combobox properly at init time

Seemed it relied on value-changed signal side-effects to be set previously.
Protect comboboxes setting from segfaults

src/bauhaus/bauhaus.c

@@ -1684,7 +1684,8 @@ void dt_bauhaus_combobox_set(GtkWidget *widget, const int pos)
   struct dt_bauhaus_widget_t *w = DT_BAUHAUS_WIDGET(widget);
   dt_bauhaus_combobox_data_t *d = &w->data.combobox;
   const int old_pos = d->active;
-  const int new_pos = CLAMP(pos, -1, (int)d->entries->len - 1);
+  const int new_pos = (d->entries) ? CLAMP(pos, -1, (int)d->entries->len - 1)
+                                   : -1;
 
   if(old_pos != new_pos)
   {

src/libs/export.c

@@ -1099,6 +1099,7 @@ void gui_init(dt_lib_module_t *self)
   for(const GList *it = darktable.imageio->plugins_format; it; it = g_list_next(it))
   {
     const dt_imageio_module_format_t *module = (dt_imageio_module_format_t *)it->data;
+    dt_bauhaus_combobox_add(d->format, module->name(module));
     if(module->widget)
     {
       gtk_container_add(GTK_CONTAINER(d->format_extra_container), module->widget);
@@ -1359,6 +1360,11 @@ void gui_init(dt_lib_module_t *self)
   const int storage_index = dt_imageio_get_index_of_storage(dt_imageio_get_storage_by_name(setting));
   dt_bauhaus_combobox_set(d->storage, storage_index);
 
+  // Set format
+  setting = dt_conf_get_string_const(CONFIG_PREFIX "format_name");
+  const int format_index = dt_imageio_get_index_of_format(dt_imageio_get_format_by_name(setting));
+  dt_bauhaus_combobox_set(d->format, format_index);
+
   dt_bauhaus_combobox_set(d->upscale, dt_conf_get_bool(CONFIG_PREFIX "upscale") ? 1 : 0);
   dt_bauhaus_combobox_set(d->high_quality, dt_conf_get_bool(CONFIG_PREFIX "high_quality_processing") ? 1 : 0);
   dt_bauhaus_combobox_set(d->export_masks, dt_conf_get_bool(CONFIG_PREFIX "export_masks") ? 1 : 0);