Native XML Plugin: Uncaught PHP error

Hi everyone,

while getting some practice for an upcoming journal migration, I toyed around with the native XML import/export plugin in OJS 3.3.0-4 and ran into the following error.

As an excercise I wanted to move an issue from JournalA to JournalB on the same OJS instance. So I successfully exported a random issue of JournalA and then I tried to import the resulting XML into JournalB:

php ./tools/importExport.php NativeImportExportPlugin import ~/journala-issue-17.xml journalb UsernameOfSiteAdmin
PHP Notice:  Only variables should be passed by reference in /var/www/html/plugins/importexport/native/filter/ on line 103
PHP Fatal error:  Uncaught Error: Call to a member function getUsername() on null in /var/www/html/lib/pkp/classes/services/
Stack trace:
#0 /var/www/html/lib/pkp/plugins/importexport/native/filter/ PKP\Services\PKPSubmissionFileService->add(Object(SubmissionFile), Object(Request))
#1 /var/www/html/lib/pkp/plugins/importexport/native/filter/ NativeXmlSubmissionFileFilter->handleElement(Object(DOMElement))
#2 /var/www/html/lib/pkp/classes/filter/ NativeImportFilter->process(Object(DOMDocument))
#3 /var/www/html/lib/pkp/plugins/importexport/native/filter/ Filter->execute(Object(DOMDocument))
#4 /var/www/html/lib/pkp/plugins/importexport/native/filter/ NativeXmlSubmissionFilter->parseSubmissionFile(Object(DOMElement), Object(Submission))
#5 /var/www/html/lib/pkp/plugins/importexport/native/filter/ in /var/www/html/lib/pkp/classes/services/ on line 321

I suspected some error with the XML, but it seems to validate:

xmllint journala-issue-17.xml --schema /var/www/html/plugins/importexport/native/native.xsd --noout
journala-issue-17.xml validates

For the sake of it, I tried another issue without any articles and this succeeded. I then tried exporting and importing two issues from and to JournalB. Again, the empty one succeeded, the other failed.

So I suppose the problem lies within the article metadata somehow. Can anyone give me a hint about what to look for? Or do I simply have a misunderstanding of what is supposed to happen?

Many thanks and best regards