fix: avoid prompt template directory error

The prompt customization service tried to access the prompt template
directory even if it did not exist (yet). This case is now checked.

Fixes #14367

packages/ai-core/src/browser/frontend-prompt-customization-service.ts

@@ -116,6 +116,10 @@ export class FrontendPromptCustomizationServiceImpl implements PromptCustomizati
         }));
 
         this.onDidChangeCustomAgentsEmitter.fire();
+
+        if (!(await this.fileService.exists(templateURI))) {
+            return;
+        }
         const stat = await this.fileService.resolve(templateURI);
         if (stat.children === undefined) {
             return;