Creating custom link resulted in "No schema found for the XXX extension point when defining extension."

There is a really good link to create custom link type by creating a new plugin project.
https://www.ibm.com/developerworks/community/blogs/extendingrtc/entry/newlink
In the above link where you add ‘com.ibm.team.repository.common.linkTypes’ in Extensions tab, it results in "No schema found for the ‘com.ibm.team.repository.common.linkTypes’ extension point when defining extension."

This issue occurs on some environment and it doesn't on other environments.
It seems like manually modifying plugin.xml resolves the issue, but the image is broken in the link and cannot add source and target point in the Extension tab.

plugin.xml looks empty.
<?xml version="1.0" encoding="UTF-8"?>
<?eclipse version="3.4"?>
<plugin>
   <extension
         point="com.ibm.team.repository.common.linkTypes">
   </extension>
</plugin>