Add Instance does not work

[seebi]

steps to have this error:

• create a new graph from SKOS model
• click on class concept
• click on "Add Instance" button (works fine here)
• add concept
• click on class concept (again)
• click on "Add Instance" button

Problem: does not work and error appears Uncaught TypeError: Cannot read property 'hashString' of null

This is maybe a OntoWiki error