Issue #3408314: Allow configuration of which vocabularies to export

Closes #3408314

src/Form/TaxonomiesSyncForm.php

@@ -95,10 +95,14 @@ public function buildForm(array $form, FormStateInterface $form_state) {
       $vocabulary_list[$vocabulary->id()] = $vocabulary->label();
     }
 
+    // This will return NULL if no data has been saved yet. So we default to an
+    // empty array, so that we can use it in '#default_value' below.
+    $taxonomies = $this->config('structure_sync.data')->get('taxonomies') ?: [];
+
     $form['export']['export_voc_list'] = [
       '#type' => 'checkboxes',
       '#options' => $vocabulary_list,
-      '#default_value' => array_keys($vocabulary_list),
+      '#default_value' => array_keys($taxonomies),
       '#title' => $this->t('Select the vocabularies you would like to export'),
     ];
 
@@ -109,8 +113,6 @@ public function buildForm(array $form, FormStateInterface $form_state) {
       '#open' => TRUE,
     ];
 
-    $taxonomies = $this->config('structure_sync.data')->get('taxonomies');
-
     if (empty($taxonomies)) {
       $form['import']['import_no_data'] = [
         '#type' => 'markup',