[BUGFIX] Use correct parents property in SelectTree component

On modernizing tree rendering, internally calculated
properties of tree nodes have been changed to start
with `__`. This patch adjusts one forgotten occurrence.

Resolves: #104052
Releases: main, 13.4
Change-Id: If5072834815b2478139abc8323fb27ca309c53d6
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84680
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Andreas Kienast <akienast@scripting-base.de>
Reviewed-by: Andreas Kienast <akienast@scripting-base.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>

Author: o-ba

Build/Sources/TypeScript/backend/form-engine/element/select-tree.ts

@@ -100,10 +100,10 @@ export class SelectTree extends Tree
    * Finds and show all parents of node
    */
   public showParents(node: any): void {
-    if (node.parents.length === 0) {
+    if (node.__parents.length === 0) {
       return;
     }
-    const parent = this.nodes[node.parents[0]];
+    const parent = this.nodes[node.__parents[0]];
     parent.__hidden = false;
     parent.__expanded = true;
     this.showParents(parent);