Add a temporary workaround for #2884870.

Author: bojanz

modules/tax/src/Form/TaxTypeForm.php

@@ -85,6 +85,7 @@ public function form(array $form, FormStateInterface $form_state) {
       '#default_value' => $plugin,
       '#required' => TRUE,
       '#disabled' => !$type->isNew(),
+      '#limit_validation_errors' => [],
       '#ajax' => [
         'callback' => '::ajaxRefresh',
         'wrapper' => $wrapper_id,

modules/tax/src/Plugin/Commerce/TaxType/Custom.php

@@ -273,6 +273,12 @@ public function removeTerritorySubmit(array $form, FormStateInterface $form_stat
    * {@inheritdoc}
    */
   public function validateConfigurationForm(array &$form, FormStateInterface $form_state) {
+    if (!isset($form['territories'])) {
+      // The form was built by a different plugin, and is now in the process
+      // of being rebuilt. Temporary workaround for #2884870.
+      return;
+    }
+
     $values = $form_state->getValue($form['#parents']);
     $values['rates'] = array_filter($values['rates'], function ($rate) {
       return !empty($rate) && !isset($rate['add_rate']);