[2.0.0-beta5] DefaultPluginManager::$moduleExtensionList must not be accessed before initialization

Problem/Motivation

2 bugs found during beta5 tests

1 A little typing error which can be fixed by this:

++ b/src/Element/ComponentElementAlter.php
@@ -56,7 +56,7 @@ class ComponentElementAlter implements TrustedCallbackInterface {
 
       // Because SDC validator is sometimes confused by an empty slot.
       // We check the current slot render element.
-      if (self::isSlotEmpty($slot)) {
+      if (is_array($slot) && self::isSlotEmpty($slot)) {
         self::mergeSlotBubbleableMetadata($element, $slot, 1);
         unset($element['#slots'][$slot_id]);
         continue;

2. A fatal error I didn't find any solution yet:
Error: Typed property Drupal\Core\Plugin\DefaultPluginManager::$moduleExtensionList must not be accessed before initialization in Drupal\ui_patterns\ComponentPluginManager->getExtensionLabel() (line 360 of modules/custom/ui_patterns/src/ComponentPluginManager.php).

They may be related to your MR of yesterday. Can you have a look?