Resolve "Introduce PSR-4 autoloading"

Closes #12

classes/Exporter.php

@@ -39,7 +39,7 @@ class Exporter
         array_push($triples, $t);
         $t['o'] = LS . 'Application';
         array_push($triples, $t);
-        if ($conf['parentApp'] != null) {
+        if (isset($conf['parentApp']) && $conf['parentApp'] !== null) {
             $t['p'] = OPMV . 'wasDerivedFrom';
             $t['o'] = $conf['parentApp'];
             array_push($triples, $t);