Add/Edit action: General error: 2006 MySQL server has gone away in

Problem/Motivation

Hi, I have a problem when add or edit an action.

The page /admin/config/workflow/business_rules/action/add show this message:

Fatal error: Uncaught PDOException: SQLSTATE[HY000]: General error: 2006 MySQL server has gone away in /var/www/html/web/core/lib/Drupal/Core/Database/StatementWrapperIterator.php:113 Stack trace: #0 /var/www/html/web/core/lib/Drupal/Core/Database/StatementWrapperIterator.php(113): PDOStatement->execute(Array) #1 /var/www/html/web/core/lib/Drupal/Core/Database/Connection.php(851): Drupal\Core\Database\StatementWrapperIterator->execute(Array, Array) #2 /var/www/html/web/core/lib/Drupal/Core/Database/Query/Select.php(524): Drupal\Core\Database\Connection->query('SELECT 1 AS "ex...', Array, Array) #3 /var/www/html/web/core/lib/Drupal/Core/Database/Query/Merge.php(379): Drupal\Core\Database\Query\Select->execute() #4 /var/www/html/web/core/lib/Drupal/Core/Session/SessionHandler.php(95): Drupal\Core\Database\Query\Merge->execute() #5 /var/www/html/web/core/lib/Drupal/Core/Session/SessionHandler.php(109): Drupal\Core\Session\SessionHandler->Drupal\Core\Session\{closure}() #6 /var/www/html/web/core/lib/Drupal/Core/Session/WriteSafeSessionHandler.php(95): Drupal\Core\Session\SessionHandler->write

Analyzing the source code, the error is generated by the getTokensLink function. The function is in ItemForm class (business_rules/src/Form/ItemForm.php).
The error is generated by the instruction:
$keyvalue->set('token_tree', $content);

When add or modify a rule, the problem is generated the second time after clear the cache.