Issue #2785717 by osman: registerNamespaces() method should check existence of `namespaces`

src/Plugin/migrate_plus/data_parser/Xml.php

@@ -300,7 +300,7 @@ class Xml extends DataParserPluginBase implements ContainerFactoryPluginInterfac
    *   The element to apply namespace registrations to.
    */
   protected function registerNamespaces(\SimpleXMLElement $xml) {
-    if (is_array($this->configuration['namespaces'])) {
+    if (isset($this->configuration['namespaces']) && is_array($this->configuration['namespaces'])) {
       foreach ($this->configuration['namespaces'] as $prefix => $ns) {
         $xml->registerXPathNamespace($prefix, $ns);
       }