OJS3 Error on the issue page

Software Support
,
1

Hello everyone!
On one of our OJS ( version 3.3.0.8 ), I have an error on some Issue view pages ( /issue/view/ ). The page rendering stops on one article title and the error I’ve found is:

Call to a member function getId() on null in 
/lib/pkp/classes/publication/PKPPublication.inc.php:168
Stack trace:
#0 /lib/pkp/classes/submission/PKPSubmission.inc.php(405): PKPPublication->getAuthorString(Array)
#1 /cache/t_compile/e26d2de610226743d9feff244fb14ef53700227f^7cf1cd52b8ba8108de77f197078de34b733dd394_0.app.frontendobjectsarticlesum.php(75): PKPSubmission->getAuthorString()
#2 /lib/pkp/lib/vendor/smarty/smarty/libs/sysplugins/smarty_template_resource_base.php(123): content_61920e77d24b91_10018494(Object(Smarty_Internal_Template))
#3 /lib/pkp/lib/vendor/smarty/smarty/libs/sysplugins/smarty_template_compiled.php(114): Smarty_Template_Resource_Base->getRenderedTemplateCode(Object(Smarty_Internal_Template))

Dumping the Array involved (the ‘$userGroups’ variable) I found this:

Array
(
    [0] => UserGroup Object
        (
            [_data] => Array
                (
                    [id] => 99
                    [roleId] => 65536
                    [contextId] => 26
                    [isDefault] => 1
                    [showTitle] => 0
                    [permitSelfRegistration] => 1
                    [permitMetadataEdit] => 0
                    [abbrevLocaleKey] => default.groups.abbrev.author
                    [nameLocaleKey] => default.groups.name.author
                    [abbrev] => Array
                        (
                            [en_US] => AU
                            [it_IT] => AU
                        )

                    [name] => Array
                        (
                            [en_US] => Author
                            [it_IT] => Autore
                        )

                )

            [_hasLoadableAdapters] => 
            [_metadataExtractionAdapters] => Array
                (
                )

            [_extractionAdaptersLoaded] => 
            [_metadataInjectionAdapters] => Array
                (
                )

            [_injectionAdaptersLoaded] => 
        )

    [1] => 
)

The only difference, with the previous articles, that I’ve noticed, is the ‘[1]’ element that is empty

Can anyone help me to solve that?
Thank you very much!