Issue #1957742 by larowlan: Fixed Can't delete a custom block instance.

core/modules/block/custom_block/lib/Drupal/custom_block/CustomBlockFormController.php

@@ -210,7 +210,7 @@ public function delete(array $form, array &$form_state) {
       unset($_GET['destination']);
     }
     $block = $this->buildEntity($form, $form_state);
-    $form_state['redirect'] = array('block/' . $block->uuid->value . '/delete', array('query' => $destination));
+    $form_state['redirect'] = array('block/' . $block->id() . '/delete', array('query' => $destination));
   }
 
 }

core/modules/block/custom_block/lib/Drupal/custom_block/Tests/PageEditTest.php

@@ -66,6 +66,11 @@ public function testPageEdit() {
     // Ensure that the block revision has been created.
     $revised_block = entity_load('custom_block', $block->id->value, TRUE);
     $this->assertNotIdentical($block->revision_id->value, $revised_block->revision_id->value, 'A new revision has been created.');
+
+    // Test deleting the block.
+    $this->drupalGet("block/" . $revised_block->id() . "/edit");
+    $this->drupalPost(NULL, array(), t('Delete'));
+    $this->assertText(format_string('Are you sure you want to delete !label?', array('!label' => $revised_block->label())));
   }
 
 }