Fix error in substituteWith if parent of node is null

Author: mhugot

src/Traits/ManipulationTrait.php

@@ -184,7 +184,9 @@ public function destroy(?string $selector = null): self {
     public function substituteWith(string|NodeList|\DOMNode|callable $input): self {
         $this->manipulateNodesWithInput($input, function($node, $newNodes) {
             foreach ($newNodes as $newNode) {
-                $node->parent()->replaceChild($newNode, $node);
+                if ($node->parent()) {
+                    $node->parent()->replaceChild($newNode, $node);
+                }
             }
         });