LibWeb: Check parent node exists before checking its type

Author: https://github.com/tcl3
Commit: d1678e03ff
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5448
Reviewed-by: https://github.com/shannonbooth ✅

Libraries/LibWeb/XML/XMLDocumentBuilder.cpp

@@ -232,7 +232,7 @@ void XMLDocumentBuilder::element_end(const XML::Name& name)
     };
 
     auto* parent = m_current_node->parent_node();
-    if (parent->is_document_fragment()) {
+    if (parent && parent->is_document_fragment()) {
         auto template_parent_node = m_template_node_stack.take_last();
         parent = template_parent_node.ptr();
     }