Revert "Issue #2819219 by BR0kEN: SystemMainBlock could lead to fatal error in...

Revert "Issue #2819219 by BR0kEN: SystemMainBlock could lead to fatal error in case if "setMainContent" method will not be called"

This reverts commit 24eeeb1b.

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}