Page Tree Gadget loads forever and does not display actual pages tree

Page tree gadget loads forever and does not show page tree (see attachment1.png).

Steps to reproduce
1. Download XWiki Graphical Installer for Windows version 7.4.5, install and run.
2. Open XWiki page and login as Admin user with the defaul password.
3. Create a new empty wiki called Test (this step is essential because the page tree gadget on dashboard of Home wiki works normally).
4. Open its dashboard.

Expected behaviuor
Page tree widget shows pages structure.

Actual bevaviour
Page tree widget shows "Loading..." message and ajax loader that spins forever.

Additional info that hopefully may help to fix the bug faster

1. The bug is probably related with js error message, see attachment1.png.
2. When opening dashboard of Home wiki that works normally it makes the following HTTP request:
http://localhost:8080/xwiki/bin/get/Dashboard/WebHome?outputSyntax=plain&sheet=XWiki.DocumentTree&showAttachments=false&showTranslations=false&&data=children&id=%23

XWiki responses with JSON array that starts like

[{"id":"document:xwiki:Main.WebHome","text":"Home"...

But on the dashboard of Test wiki the request URL is
http://localhost:8080/xwiki/wiki/test/get/WikiManager/Dashboard?outputSyntax=plain&sheet=XWiki.DocumentTree&showAttachments=false&showTranslations=false&&data=children&id=%23

and the response is just a plane text like this:

Wiki Information

Welcome to the Test wiki.You are the owner of this wiki.
...

The script tree.min.js of xwiki probably expects array but receives this text instead (see attachment2.png).