TypeError: Argument 3 passed to MiddlewareDispatcher::beforeOutput() must be of the type string, null given, called in ....Http/Dispatcher.php on line 123

Describe the bug
When visiting the bookmarks section, the following error is reported in Nextcloud.log:

TypeError: Argument 3 passed to OC\AppFramework\Middleware\MiddlewareDispatcher::beforeOutput() must be of the type string, null given, called in ....nextcloud-18.0.7/lib/private/AppFramework/Http/Dispatcher.php on line 123

To Reproduce
Steps to reproduce the behavior:

1. Go to 'settings/admin/logging' in one tab (Firefox)
2. Go to 'apps/bookmarks' in another tab (Chrome)
3. See error in the logging

Expected behavior
No error in the log file

Screenshots
Not applicable

Desktop (please complete the following information):

• OS: Linux openSUSE_Leap_15.2
• Browser Chrome, Version 84.0
• Browser Firefox, Version 78

Server (please complete the following information):

• OS: openSUSE_Leap_15.2
• HTTP server: apache 2.4.43
• Database sqlite3
• PHP version: 7.4
• Nextcloud version: 18.0.7
• Bookmarks app version: [e.g. 1.1.0 (see Nextcloud apps page)]
• Activated Nextcloud Apps: [the output of running $ sudo -u www-data php occ app:list in your command line]

Enabled:

• accessibility: 1.4.0
• activity: 2.11.0
• admin_audit: 1.8.0
• apporder: 0.10.0
• bookmarks: 3.3.3
• bruteforcesettings: 1.6.0
• calendar: 2.0.3
• circles: 0.18.9
• cloud_federation_api: 1.1.0
• comments: 1.8.0
• contacts: 3.3.0
• cookbook: 0.7.6
• cospend: 1.0.5
• data_request: 1.5.0
• dav: 1.14.0
• deck: 1.0.5
• drop_account: 0.2.1
• external: 3.5.0
• federatedfilesharing: 1.8.0
• federation: 1.8.0
• files: 1.13.1
• files_external: 1.9.0
• files_pdfviewer: 1.7.0
• files_rightclick: 0.15.2
• files_sharing: 1.10.1
• files_trashbin: 1.8.0
• files_videoplayer: 1.7.0
• firstrunwizard: 2.7.0
• logreader: 2.3.0
• lookup_server_connector: 1.6.0
• nextcloud_announcements: 1.7.0
• notes: 3.6.1
• notifications: 2.6.0
• oauth2: 1.6.0
• password_policy: 1.8.0
• photos: 1.0.0
• polls: 1.4.3
• previewgenerator: 2.3.0
• privacy: 1.2.0
• provisioning_api: 1.8.0
• qownnotesapi: 20.1.0
• quota_warning: 1.7.0
• recommendations: 0.6.0
• serverinfo: 1.8.0
• settings: 1.0.0
• sharebymail: 1.8.0
• support: 1.1.1
• survey_client: 1.6.0
• systemtags: 1.8.0
• text: 2.0.0
• theming: 1.9.0
• twofactor_backupcodes: 1.7.0
• updatenotification: 1.8.0
• viewer: 1.2.0
• workflowengine: 2.0.0

Additional context

• Watching the logfile and going to the other apps does not result in an error.
  The only app showing the error is bookmarks
  The error is shown 4 times in a row (always 4 times)

Web server error log

Insert your webserver log here

Nextcloud log (nextcloud/data/nextcloud.log)

TypeError: Argument 3 passed to OC\AppFramework\Middleware\MiddlewareDispatcher::beforeOutput() must be of the type string, null given, called in /srv/www/vhosts/owncloud.radoeka.nl/nextcloud-18.0.7/lib/private/AppFramework/Http/Dispatcher.php on line 123

/srv/www/vhosts/owncloud.radoeka.nl/nextcloud-18.0.7/lib/private/AppFramework/Http/Dispatcher.php - line 123:
OC\AppFramework\Middleware\MiddlewareDispatcher->beforeOutput()

/srv/www/vhosts/owncloud.radoeka.nl/nextcloud-18.0.7/lib/private/AppFramework/App.php - line 125:
OC\AppFramework\Http\Dispatcher->dispatch()

/srv/www/vhosts/owncloud.radoeka.nl/nextcloud-18.0.7/lib/private/AppFramework/Routing/RouteActionHandler.php - line 47:
OC\AppFramework\App::main()

<<closure>>
OC\AppFramework\Routing\RouteActionHandler->__invoke()

/srv/www/vhosts/owncloud.radoeka.nl/nextcloud-18.0.7/lib/private/Route/Router.php - line 299:
call_user_func()

/srv/www/vhosts/owncloud.radoeka.nl/nextcloud-18.0.7/lib/base.php - line 1008:
OC\Route\Router->match()

/srv/www/vhosts/owncloud.radoeka.nl/nextcloud-18.0.7/index.php - line 38:
OC::handleRequest()```

Start with lib/private/AppFramework/Http/Dispatcher.php line 123 and the MiddlewareDispatcher::beforeOutput() call shown in the trace, then compare the bookmarks request with the other apps that do not log the error. Reproduce by opening apps/bookmarks while watching the Nextcloud log. Done means the bookmarks page no longer produces the TypeError.