Hello Maxime,
Are you using Eclipse Mars?
If yes, there is a bug with this version. Take a look
here
http://stackoverflow.com/questions/31826410/diagramtypeprovider-must-not-be-null
and here
https://bugs.eclipse.org/bugs/show_bug.cgi?id=472835
Please try to use Eclipse Luna or the workaround indicated in the link. It should work properly.
Please let me know the results and reward if the answer was helpful.
Regards,
Diego Gabriel