ERROR while accessing joints

Hi there,

I was trying to load gazebo with a world file. But it gives following error and exits.

**[Err] [Diff.cc:41] Diff plugin missing <left_joint> element
[Err] [Diff.cc:44] Diff plugin missing <right_joint> element
Error [Element.cc:671] Missing element description for [right_joint]
gzserver: /usr/include/boost/smart_ptr/shared_ptr.hpp:648: typename boost::detail::sp_member_access::type boost::shared_ptr::operator->() const [with T = sdf::Element; typename boost::detail::sp_member_access::type = sdf::Element*]: Assertion `px != 0' failed.
Aborted**

In the world file I've specified left_element and right_element. Why this is happening?
Also, what is meant by **Missing element description for [right_joint]**? I didn't find any description for joints anywhere. Can anyone provide a solution for this?

Files:
1. [World file](http://pastebin.com/nhUyEAKM)
2. [Model.sdf](http://pastebin.com/cJV19UMh)
3. [Model.config](http://pastebin.com/vXRdmXM7)
4. [Plugin](http://pastebin.com/gpQSXAMk)

Thanks in advance!