Error on linking a category of another language

This PR for fix issue #4467 

## Original Report

#### Steps to reproduce the issue
create a content category for a language.
create another content category for the second langauge.
link the two categories with each other.

#### Expected result
linked categories


#### Actual result
errors in linking popup:
Warning: Attempt to assign property of non-object in /srv/www/vhosts/kubler.ch/httpdocs/libraries/framework/Joomla/Registry/Registry.php on line 428

Warning: Attempt to assign property of non-object in /srv/www/vhosts/kubler.ch/httpdocs/libraries/framework/Joomla/Registry/Registry.php on line 428

Warning: Attempt to assign property of non-object in /srv/www/vhosts/kubler.ch/httpdocs/libraries/framework/Joomla/Registry/Registry.php on line 428


#### System information (as much as possible)
joomla 3.3.6


#### Additional comments
sometimes the linking works, but on some parcticular categories i get the errors above.