I can replicate it with Smart content module of 3.1.0 module
{ "message": "\nAn AJAX HTTP error occurred.\nHTTP Result Code: 200\nDebugging information follows.\nPath: /admin/structure/block/add/smart_content_decision_block/convivial_bootstrap?region=sidebar&_wrapper_format=drupal_ajax&ajax_form=1\nStatusText: parsererror\nResponseText: TypeError: Drupal\\Core\\Render\\MainContent\\AjaxRenderer::renderResponse(): Argument #1 ($main_content) must be of type array, null given, called in /app/web/core/lib/Drupal/Core/Form/FormAjaxResponseBuilder.php on line 89 in Drupal\\Core\\Render\\MainContent\\AjaxRenderer->renderResponse() (line 49 of /app/web/core/lib/Drupal/Core/Render/MainContent/AjaxRenderer.php).", "name": "AjaxError" }
Moving to smart_content module. Seeking advice on how to fix it