Fix: Stop using null as array offset (Resolves #227)

Author: lacatoire

phpdotnet/phd/Package/PHP/Web.php

@@ -204,7 +204,8 @@ public function header($id) {
             "alternatives" => $this->cchunk["alternatives"],
             "source" => $this->sourceInfo($id),
         );
-        $setup["history"] = $this->history[$setup["source"]["path"]] ?? [];
+        $history = $this->history ?? [];
+        $setup["history"] = $history[$setup["source"]["path"]] ?? [];
         if ($this->getChildren($id)) {
             $lang = $this->config->language;
             $setup["extra_header_links"] = array(