.../BlockPluginHasOffCanvasFormAccessCheck.php     |  3 +-
 ...OffCanvasFormAccessCheckTestFormsWithForms.php} | 80 +++++++++++++++-------
 2 files changed, 56 insertions(+), 27 deletions(-)

diff --git a/core/modules/outside_in/src/Access/BlockPluginHasOffCanvasFormAccessCheck.php b/core/modules/outside_in/src/Access/BlockPluginHasOffCanvasFormAccessCheck.php
index 6966943..e8136d6 100644
--- a/core/modules/outside_in/src/Access/BlockPluginHasOffCanvasFormAccessCheck.php
+++ b/core/modules/outside_in/src/Access/BlockPluginHasOffCanvasFormAccessCheck.php
@@ -5,6 +5,7 @@
 use Drupal\block\BlockInterface;
 use Drupal\Core\Access\AccessResult;
 use Drupal\Core\Block\BlockPluginInterface;
+use Drupal\Core\Plugin\PluginWithFormsInterface;
 use Drupal\Core\Routing\Access\AccessInterface;
 
 /**
@@ -42,7 +43,7 @@ public function access(BlockInterface $block) {
    * @todo Remove when outside_in_preprocess_block() is removed.
    */
   public function accessBlockPlugin(BlockPluginInterface $block_plugin) {
-    return AccessResult::allowedIf($block_plugin->hasFormClass('off_canvas'));
+    return AccessResult::allowedIf($block_plugin instanceof PluginWithFormsInterface && $block_plugin->hasFormClass('off_canvas'));
   }
 
 }
diff --git a/core/modules/outside_in/tests/src/Unit/Access/BlockPluginHasOffCanvasFormAccessCheckTest.php b/core/modules/outside_in/tests/src/Unit/Access/BlockPluginHasOffCanvasFormAccessCheckTestFormsWithForms.php
similarity index 26%
rename from core/modules/outside_in/tests/src/Unit/Access/BlockPluginHasOffCanvasFormAccessCheckTest.php
rename to core/modules/outside_in/tests/src/Unit/Access/BlockPluginHasOffCanvasFormAccessCheckTestFormsWithForms.php
index 6d5366b..8456f13 100644
--- a/core/modules/outside_in/tests/src/Unit/Access/BlockPluginHasOffCanvasFormAccessCheckTest.php
+++ b/core/modules/outside_in/tests/src/Unit/Access/BlockPluginHasOffCanvasFormAccessCheckTestFormsWithForms.php
@@ -7,6 +7,11 @@
 use Drupal\Core\Access\AccessResultInterface;
 use Drupal\Core\Access\AccessResultNeutral;
 use Drupal\Core\Block\BlockBase;
+use Drupal\Core\Block\BlockPluginInterface;
+use Drupal\Core\Cache\UnchangingCacheableDependencyTrait;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Plugin\PluginWithFormsInterface;
+use Drupal\Core\Session\AccountInterface;
 use Drupal\outside_in\Access\BlockPluginHasOffCanvasFormAccessCheck;
 use Drupal\Tests\UnitTestCase;
 
@@ -21,8 +26,8 @@ class BlockPluginHasOffCanvasFormAccessCheckTest extends UnitTestCase {
    * @covers ::accessBlockPlugin
    * @dataProvider providerTestAccess
    */
-  public function testAccess(array $plugin_definition, AccessResultInterface $expected_access_result) {
-    $block_plugin = new TestBlockClass([], $this->randomMachineName(), $plugin_definition);
+  public function testAccess($class, array $plugin_definition, AccessResultInterface $expected_access_result) {
+    $block_plugin = new $class([], $this->randomMachineName(), $plugin_definition);
     $block = $this->prophesize(BlockInterface::class);
     $block->getPlugin()->willReturn($block_plugin);
 
@@ -32,40 +37,63 @@ public function testAccess(array $plugin_definition, AccessResultInterface $expe
   }
 
   public function providerTestAccess() {
-    return [
-      'set to class' => [
-        [
-          'provider' => 'block_test',
-          'forms' => [
-            'off_canvas' => $this->randomMachineName(),
-          ],
-        ],
-        new AccessResultAllowed(),
-      ],
-      'not set' => [
-        [
-          'provider' => 'block_test',
-        ],
-        new AccessResultNeutral(),
+    $annotation_forms_off_canvas_class = [
+      'provider' => 'block_test',
+      'forms' => [
+        'off_canvas' => $this->randomMachineName(),
       ],
-      'set to FALSE' => [
-        [
-          'provider' => 'block_test',
-          'forms' => [
-            'off_canvas' => FALSE,
-          ],
-        ],
-        new AccessResultNeutral(),
+    ];
+    $annotation_forms_off_canvas_not_set = [
+      'provider' => 'block_test',
+    ];
+    $annotation_forms_off_canvas_false = [
+      'provider' => 'block_test',
+      'forms' => [
+        'off_canvas' => FALSE,
       ],
     ];
+    return [
+      'block plugin with forms, forms[off_canvas] set to class' => [TestBlockClassWithForms::class, $annotation_forms_off_canvas_class, new AccessResultAllowed()],
+      'block plugin with forms, forms[off_canvas] not set' => [TestBlockClassWithForms::class, $annotation_forms_off_canvas_not_set, new AccessResultNeutral()],
+      'block plugin with forms, forms[off_canvas] set to FALSE' => [TestBlockClassWithForms::class, $annotation_forms_off_canvas_false, new AccessResultNeutral()],
+      // In practice, all block plugins extend BlockBase, which means they all
+      // implement PluginWithFormsInterface, but this may change in the future.
+      // This ensures Settings Tray will continue to work correctly.
+      'block plugin without forms, forms[off_canvas] set to class' => [TestBlockClassWithoutForms::class, $annotation_forms_off_canvas_class, new AccessResultNeutral()],
+      'block plugin without forms, forms[off_canvas] not set' => [TestBlockClassWithoutForms::class, $annotation_forms_off_canvas_not_set, new AccessResultNeutral()],
+      'block plugin without forms, forms[off_canvas] set to FALSE' => [TestBlockClassWithoutForms::class, $annotation_forms_off_canvas_false, new AccessResultNeutral()],
+    ];
   }
 
 }
 
-class TestBlockClass extends BlockBase {
+class TestBlockClassWithForms extends BlockBase {
 
   public function build() {
     return [];
   }
 
 }
+
+class TestBlockClassWithoutForms implements BlockPluginInterface {
+  use UnchangingCacheableDependencyTrait;
+  public function label() {}
+  public function access(AccountInterface $account, $return_as_object = FALSE) {}
+  public function build() {}
+  public function setConfigurationValue($key, $value) {}
+  public function blockForm($form, FormStateInterface $form_state) {}
+  public function blockValidate($form, FormStateInterface $form_state) {}
+  public function blockSubmit($form, FormStateInterface $form_state) {}
+  public function getMachineNameSuggestion() {}
+  public function getConfiguration() {}
+  public function setConfiguration(array $configuration) {}
+  public function defaultConfiguration() {}
+  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {}
+  public function validateConfigurationForm(array &$form, FormStateInterface $form_state) {}
+  public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {}
+  public function getPluginId() {}
+  public function getPluginDefinition() {}
+  public function getBaseId() {}
+  public function getDerivativeId() {}
+  public function calculateDependencies() {}
+}