In posting.php while checking the topic type to determine if the user is authorized to use the topic type they are requesting, and an extension adds an additional topic type, an invalid blank value will be sent to acl_get() per the default option in the switch statement:
https://github.com/phpbb/phpbb/blob/develop-ascraeus/phpBB/posting.php#L1124

IMO the following line where it invokes acl_get() should be prepended with a check that $auth_option in not empty. Otherwise it is impossible for extensions to add topic types. The extension author is responsible for checking the permission for this separately (and can be achieved by nearby event calls).