core/lib/Drupal/Core/Block/BlockBase.php | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/core/lib/Drupal/Core/Block/BlockBase.php b/core/lib/Drupal/Core/Block/BlockBase.php
index bcc3954..cac676e 100644
--- a/core/lib/Drupal/Core/Block/BlockBase.php
+++ b/core/lib/Drupal/Core/Block/BlockBase.php
@@ -277,7 +277,9 @@ public function setTransliteration(TransliterationInterface $transliteration) {
    */
   public function getFormClass($operation) {
     if ($this->hasFormClass($operation)) {
-      return $this->getPluginDefinition()['forms'][$operation];
+      $plugin_definition = $this->getPluginDefinition();
+      // Fallback to the class of this plugin if the key is not set.
+      return isset($plugin_definition['forms'][$operation]) ? $plugin_definition['forms'][$operation] : get_class($this);
     }
   }

@@ -285,7 +287,8 @@ public function getFormClass($operation) {
    * {@inheritdoc}
    */
   public function hasFormClass($operation) {
-    return isset($this->getPluginDefinition()['forms'][$operation]);
+    return isset($this->getPluginDefinition()['forms'][$operation]) || ($operation == 'configure'
+      && $this instanceof PluginFormInterface);
   }
 
 }