diff --git a/core/modules/system/src/Plugin/Block/SystemMainBlock.php b/core/modules/system/src/Plugin/Block/SystemMainBlock.php
index beba38d..d54a1ca 100644
--- a/core/modules/system/src/Plugin/Block/SystemMainBlock.php
+++ b/core/modules/system/src/Plugin/Block/SystemMainBlock.php
@@ -20,7 +20,7 @@ class SystemMainBlock extends BlockBase implements MainContentBlockPluginInterfa
    *
    * @var array
    */
-  protected $mainContent;
+  protected $mainContent = [];
 
   /**
    * {@inheritdoc}