[3.x]: TypeError: Return value of craft\fields\Assets::_findFolder()

What happened?

Description

Just updated a Craft CMS 3 site to 3.9.5 and now getting this error when trying to create a new entry:

TypeError: Return value of craft\fields\Assets::_findFolder() must be an instance of craft\models\VolumeFolder, null returned

FWIW the entry type does have a couple of asset fields, which "Restrict assets to a single folder" on Amazon S3 and that asset location does contain {slug} (I wonder if that might be of importance).

I can create new entries in other sections without any problems.

The only slightly awkward point is that I can't reproduce this issue locally, only in production, which is unusual. But given this site isn't under active development and the Craft CMS upgrade is the only change that's been made (this was working just fine a few days ago) I'm a little out of ideas!

Steps to reproduce

1. Go to a section and hit the "New entry" button.
2. Get a 500 error.

Stack trace

TypeError: Return value of craft\fields\Assets::_findFolder() must be an instance of craft\models\VolumeFolder, null returned
(/domain.com/vendor/craftcms/cms/src/fields/Assets.php:889)
        return $folder;
craft\fields\Assets::_findFolder(/domain.com/vendor/craftcms/cms/src/fields/Assets.php:947)
            return $this->_findFolder($sourceKey, $subpath, $element, $createDynamicFolders);
craft\fields\Assets::_uploadFolder(/domain.com/vendor/craftcms/cms/src/fields/Assets.php:638)
        $folder = $this->_uploadFolder($element, false);
craft\fields\Assets::inputSources(/domain.com/vendor/craftcms/cms/src/fields/BaseRelationField.php:1008)
            'sources' => $this->inputSources($element),
craft\fields\BaseRelationField::inputTemplateVariables(/domain.com/vendor/craftcms/cms/src/fields/Assets.php:698)
        $variables = parent::inputTemplateVariables($value, $element);
craft\fields\Assets::inputTemplateVariables(/domain.com/vendor/craftcms/cms/src/fields/BaseRelationField.php:594)
        $variables = $this->inputTemplateVariables($value, $element);
craft\fields\BaseRelationField::inputHtml(/domain.com/vendor/craftcms/cms/src/fields/Assets.php:283)
            return parent::inputHtml($value, $element);
craft\fields\Assets::inputHtml(/domain.com/vendor/craftcms/cms/src/base/Field.php:465)
        $html = $this->inputHtml($value, $element);
craft\base\Field::getInputHtml(/domain.com/vendor/craftcms/cms/src/fieldlayoutelements/CustomField.php:260)
        $html = $this->_field->getInputHtml($value, $element);
craft\fieldlayoutelements\CustomField::inputHtml(/domain.com/vendor/craftcms/cms/src/fieldlayoutelements/BaseField.php:203)
        $inputHtml = $this->inputHtml($element, $static);
craft\fieldlayoutelements\BaseField::formHtml(/domain.com/vendor/craftcms/cms/src/fieldlayoutelements/CustomField.php:213)
            return (string)parent::formHtml($element, $static);
craft\fieldlayoutelements\CustomField::craft\fieldlayoutelements\{closure}(/domain.com/vendor/craftcms/cms/src/web/View.php:1529)
            $response = $this->namespaceInputs($html(), $namespace, $otherAttributes, $withClasses);
craft\web\View::namespaceInputs(/domain.com/vendor/craftcms/cms/src/fieldlayoutelements/CustomField.php:214)
        }, 'fields');
craft\fieldlayoutelements\CustomField::formHtml(/domain.com/vendor/craftcms/cms/src/models/FieldLayout.php:594)
                    return (string)$layoutElement->formHtml($element, $static);
craft\models\FieldLayout::craft\models\{closure}(/domain.com/vendor/craftcms/cms/src/web/View.php:1524)
                return $html();
craft\web\View::namespaceInputs(/domain.com/vendor/craftcms/cms/src/models/FieldLayout.php:593)
                $elementHtml = $view->namespaceInputs(function() use ($layoutElement, $element, $static) {
craft\models\FieldLayout::createForm(/domain.com/vendor/twig/twig/src/Extension/CoreExtension.php:1570)
        $ret = $object->$method(...$arguments);
twig_get_attribute(/domain.com/vendor/craftcms/cms/src/helpers/Template.php:106)
            return \twig_get_attribute($env, $source, $object, $item, $arguments, $type, $isDefinedTest, $ignoreStrictCheck);
craft\helpers\Template::attribute(/domain.com/storage/runtime/compiled_templates/cc/ccdeb57a8125a11059a9d998753a3fd8fa9394a42559dec1c7d29839d543e952.php:175)
        $context["form"] = craft\helpers\Template::attribute($this->env, $this->source, craft\helpers\Template::attribute($this->env, $this->source, ($context["element"] ?%3F_null%29%2C_%22getFieldLayout%22%2C_%5B0%5D=%3E+%28%24context%5B%22element%22%5D+%
__TwigTemplate_77a0c1f2083b9dfb852c9a7f66358708c8549cc77bf8ab70910e2ddfc83e2509::doDisplay(/domain.com/vendor/twig/twig/src/Template.php:405)
            $this->doDisplay($context, $blocks);
Twig\Template::displayWithErrorHandling(/domain.com/vendor/twig/twig/src/Template.php:378)
        $this->displayWithErrorHandling($this->env->mergeGlobals($context), array_merge($this->blocks, $blocks));
Twig\Template::display(/domain.com/storage/runtime/compiled_templates/6f/6f077381a6ddf3dcae3da5b6ea2777fefc054070a084d88ebfbf1f2933986d6b.php:124)
        $this->parent->display($context, array_merge($this->blocks, $blocks));
__TwigTemplate_367b54d6e17f226ad63b87287416126ede1b4df71984d3ffd174f3ecde983176::doDisplay(/domain.com/vendor/twig/twig/src/Template.php:405)
            $this->doDisplay($context, $blocks);
Twig\Template::displayWithErrorHandling(/domain.com/vendor/twig/twig/src/Template.php:378)
        $this->displayWithErrorHandling($this->env->mergeGlobals($context), array_merge($this->blocks, $blocks));
Twig\Template::display(/domain.com/vendor/twig/twig/src/Template.php:390)
            $this->display($context);
Twig\Template::render(/domain.com/vendor/twig/twig/src/TemplateWrapper.php:45)
        return $this->template->render($context, \func_get_args()[1] ?%3F_%5B0%5D=
Twig\TemplateWrapper::render(/domain.com/vendor/twig/twig/src/Environment.php:318)
        return $this->load($name)->render($context);
Twig\Environment::render(/domain.com/vendor/craftcms/cms/src/web/View.php:408)
            $output = $this->getTwig()->render($template, $variables);
craft\web\View::renderTemplate(/domain.com/vendor/craftcms/cms/src/web/View.php:461)
            echo $this->renderTemplate($template, $variables);
craft\web\View::renderPageTemplate(/domain.com/vendor/craftcms/cms/src/web/Controller.php:216)
        $this->response->data = $view->renderPageTemplate($template, $variables, $templateMode);
craft\web\Controller::renderTemplate(/domain.com/vendor/craftcms/cms/src/controllers/EntriesController.php:258)
        return $this->renderTemplate('entries/_edit', $variables);
craft\controllers\EntriesController::actionEditEntry(:)

call_user_func_array(/domain.com/vendor/yiisoft/yii2/base/InlineAction.php:57)
        return call_user_func_array([$this->controller, $this->actionMethod], $args);
yii\base\InlineAction::runWithParams(/domain.com/vendor/yiisoft/yii2/base/Controller.php:178)
            $result = $action->runWithParams($params);
yii\base\Controller::runAction(/domain.com/vendor/yiisoft/yii2/base/Module.php:552)
            $result = $controller->runAction($actionID, $params);
yii\base\Module::runAction(/domain.com/vendor/craftcms/cms/src/web/Application.php:295)
        $result = parent::runAction($route, $params);
craft\web\Application::runAction(/domain.com/vendor/yiisoft/yii2/web/Application.php:103)
            $result = $this->runAction($route, $params);
yii\web\Application::handleRequest(/domain.com/vendor/craftcms/cms/src/web/Application.php:280)
            return parent::handleRequest($request);
craft\web\Application::handleRequest(/domain.com/vendor/yiisoft/yii2/base/Application.php:384)
            $response = $this->handleRequest($this->getRequest());
yii\base\Application::run(/sites/domain.com/public/index.php:26)
$app->run();

Craft CMS version

3.9.5

PHP version

7.3.33

Operating system and version

Ubuntu 20.04

Database type and version

MySQL 5.7

Image driver and version

No response

Installed plugins and versions

Plugin	Version
Amazon S3	1.3.2
Asset Rev	6.0.2
Embedded Assets	2.11.4
Expanded Singles	1.2.0
Image Resizer	2.2.2
Maps	3.9.4
MatrixMate	1.5.0
Phone Number	1.5.0
Redactor	2.10.12
Rollbar	1.0.3
SEO	3.4.62
Super Table	2.7.5.1
Typed link field	2.0.0-rc.2
Webhooks	2.4.1