Issue #3465497 by grimreaper, pdureau: Ensure slotsAlter exists to avoid fatal error.

src/Element/ComponentElementBuilder.php

@@ -50,6 +50,7 @@ class ComponentElementBuilder implements TrustedCallbackInterface {
     $element = $this->buildProps($element, $component, $configuration, $contexts);
     $element = $this->buildSlots($element, $component, $configuration, $contexts);
     $element['#propsAlter'] = [];
+    $element['#slotsAlter'] = [];
     return $element;
   }