Refactor parent template handling in TemplateManager

- Updated the logic to retrieve and validate the parent template content, ensuring it is correctly fetched and checked for string type.
- Improved error logging to provide clearer messages when parent template content is missing or invalid.

src/templates/templatemanager.js

@@ -427,24 +427,37 @@ export class TemplateManager {
 
       var templateWithParentContent;
 
+      var parentTemplate = documentTemplate.parent;
+
       if (documentTemplate.parent != undefined) {
+        if (typeof parentTemplate === 'string') {
+          parentTemplate = await getObject({
+            model: documentTemplateModel,
+            id: parentTemplate,
+            populate: [
+              { path: 'documentSize' },
+              { path: 'parent', strictPopulate: false }
+            ],
+            cached: true
+          });
+        }
         // Validate parent content
         if (
-          documentTemplate.parent.content == null ||
+          parentTemplate.content == null ||
-          typeof documentTemplate.parent.content !== 'string'
+          typeof parentTemplate.content !== 'string'
         ) {
           logger.error(
             'Parent template content is required and must be a string.',
-            documentTemplate.parent.content
+            parentTemplate.content
           );
           return {
             error:
               'Parent template content is required and must be a string.' +
-              documentTemplate.parent.content
+              parentTemplate.content
           };
         }
         templateWithParentContent = await ejs.render(
-          documentTemplate.parent.content,
+          parentTemplate.content,
           { content: templateContent, fc: this.fc },
           defaultOptions
         );