>>> [!note] Migrated issue <!-- Drupal.org comment --> <!-- Migrated from issue #3551842. --> Reported by: [mdranove](https://www.drupal.org/user/3695874) Related to !125 !124 >>> <h3 id="summary-problem-motivation">Problem/Motivation</h3> <p>If you go to the book settings page, add a new allowed type, but do not select anything for the allowed children, save, and then try to save the settings page again, an exception is thrown.</p> <h4 id="summary-steps-reproduce">Steps to reproduce</h4> <p>On fresh d11 book 3.x install</p> <p>Navigate to /admin/structure/book/settings.</p> <ol> <li>Under Allowed content types and their children, choose a new type, but do not select anything for the allowed child type.</li> <li>Save.</li> <li>Go back to the settings page.</li> <li>Try to save again.</li> <li>An exception is thrown.</li> </ol> <h3 id="summary-proposed-resolution">Proposed resolution</h3> <p>The problem is that "null" is being saved to the config for allowed child types but a string is expected.</p> <h3 id="summary-remaining-tasks">Remaining tasks</h3> <ul> <li>Add validate() to settings form to check that each allowed content_type has a corresponding allowed child_type.</li> <li>Add NotNull constraint to schema.</li> <li>Add tests.</li> <li>Add post update hook.</li> </ul> <h3 id="summary-ui-changes">User interface changes</h3> <h3 id="summary-api-changes">API changes</h3> <h3 id="summary-data-model-changes">Data model changes</h3>