[5.x] Cache structure on nav tree instance (#9648)

Co-authored-by: Jason Varga <jason@pixelfear.com>

src/Structures/NavTree.php

@@ -14,9 +14,15 @@
 
 class NavTree extends Tree implements TreeContract
 {
+    private $structureCache;
+
     public function structure()
     {
-        return Blink::once('nav-tree-structure-'.$this->handle(), function () {
+        if ($this->structureCache) {
+            return $this->structureCache;
+        }
+
+        return $this->structureCache = Blink::once('nav-tree-structure-'.$this->handle(), function () {
             return Nav::findByHandle($this->handle());
         });
     }