Make sure context_mapping information is stored in BlockBase::submitConfigurationForm

Closes #3251378

core/lib/Drupal/Core/Block/BlockBase.php

@@ -23,6 +23,7 @@ abstract class BlockBase extends PluginBase implements BlockPluginInterface, Plu
 
   use BlockPluginTrait {
     buildConfigurationForm as traitBuildConfigurationForm;
+    submitConfigurationForm as traitSubmitConfigurationForm;
   }
   use ContextAwarePluginTrait;
   use ContextAwarePluginAssignmentTrait;
@@ -40,4 +41,14 @@ public function buildConfigurationForm(array $form, FormStateInterface $form_sta
     return $form;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {
+    if (!$form_state->getErrors() && $form_state->getValue('context_mapping')) {
+      $this->configuration['context_mapping'] = $form_state->getValue('context_mapping');
+    }
+    $this->traitSubmitConfigurationForm($form, $form_state);
+  }
+
 }