How to reeproduce:
Change XML to import and add a nature that is not present in the XML <natures>
Then import it:
error [XML import][
art #1469] Nature test1233 not found on plateform. Artifact 1467 added without nature.
PHP Notice: Array to string conversion in /usr/share/tuleap/plugins/tracker/include/Tracker/Artifact/XMLImport/ArtifactFieldsDataBuilder.class.php on line 157