Issue #3274246 by Matroskeen: Fix property types in EntityLookup

src/Plugin/migrate/process/EntityLookup.php

@@ -97,7 +97,7 @@ class EntityLookup extends ProcessPluginBase implements ContainerFactoryPluginIn
   protected ?string $destinationBundleKey = NULL;
   protected ?string $lookupValueKey = NULL;
   protected ?string $lookupBundleKey = NULL;
-  protected ?string $lookupBundle = NULL;
+  protected mixed $lookupBundle = NULL;
   protected ?string $lookupEntityType = NULL;
   protected ?string $destinationProperty;
   protected bool $accessCheck = TRUE;