diff --git a/src/Entity/PageVariant.php b/src/Entity/PageVariant.php
index adbd602..e72bc99 100644
--- a/src/Entity/PageVariant.php
+++ b/src/Entity/PageVariant.php
@@ -117,9 +117,9 @@ class PageVariant extends ConfigEntityBase implements PageVariantInterface {
   /**
    * An array of collected contexts.
    *
-   * @var \Drupal\Component\Plugin\Context\ContextInterface[]|null
+   * @var \Drupal\Component\Plugin\Context\ContextInterface[]
    */
-  protected $contexts = NULL;
+  protected $contexts = [];
 
   /**
    * Static context references.
@@ -277,7 +277,7 @@ class PageVariant extends ConfigEntityBase implements PageVariantInterface {
    * @return $this
    */
   protected function resetCollectedContexts() {
-    $this->contexts = NULL;
+    $this->contexts = [];
     return $this;
   }
 
@@ -447,7 +447,10 @@ class PageVariant extends ConfigEntityBase implements PageVariantInterface {
   public function __sleep() {
     $vars = parent::__sleep();
     // Gathered contexts objects should not be serialized.
-    unset($vars[array_search('contexts', $vars)]);
+    $key = array_search('contexts', $vars);
+    if ($key !== FALSE) {
+      unset($vars[$key]);
+    }
     return $vars;
   }
 
diff --git a/src/Plugin/DisplayVariant/HttpStatusCodeDisplayVariant.php b/src/Plugin/DisplayVariant/HttpStatusCodeDisplayVariant.php
index ba701c0..db5d6b7 100644
--- a/src/Plugin/DisplayVariant/HttpStatusCodeDisplayVariant.php
+++ b/src/Plugin/DisplayVariant/HttpStatusCodeDisplayVariant.php
@@ -86,7 +86,10 @@ class HttpStatusCodeDisplayVariant extends VariantBase {
 
   public function __sleep() {
     $vars = parent::__sleep();
-    unset($vars[array_search('stringTranslation', $vars)]);
+    $key = array_search('stringTranslation', $vars);
+    if ($key !== FALSE) {
+      unset($vars[$key]);
+    }
     return $vars;
   }
 
diff --git a/src/Plugin/DisplayVariant/PageBlockDisplayVariant.php b/src/Plugin/DisplayVariant/PageBlockDisplayVariant.php
index 2f8e257..263fbf2 100644
--- a/src/Plugin/DisplayVariant/PageBlockDisplayVariant.php
+++ b/src/Plugin/DisplayVariant/PageBlockDisplayVariant.php
@@ -253,7 +253,7 @@ class PageBlockDisplayVariant extends BlockDisplayVariant implements PluginWizar
     return [
       'content' => [
         'title' => $this->t('Content'),
-        'form' => '\Drupal\page_manager\Form\VariantPluginContentForm',
+        'form' => '\Drupal\page_manager_ui\Form\VariantPluginContentForm',
       ],
     ];
   }
@@ -315,7 +315,10 @@ class PageBlockDisplayVariant extends BlockDisplayVariant implements PluginWizar
 
   public function __sleep() {
     $vars = parent::__sleep();
-    unset($vars[array_search('stringTranslation', $vars)]);
+    $key = array_search('stringTranslation', $vars);
+    if ($key !== FALSE) {
+      unset($vars[$key]);
+    }
     return $vars;
   }