Trying to add a subclass on the knowledge base page creates an error

[reckart]

Describe the bug
I wanted to add 'Ischemic Nephropathy' as a subclass of 'Nephropathy' in my HPO Knowledgebase. I searched for 'Nephropathy' in the Knowledge Base, then clicked on the 'New Subclass" button and entered 'Ischemic Nephropathy'. When I click 'Create' I get a 'Whoops' error (see screenshot below). Strangely, when I go back to the Knowledge Base and search for 'Ischemic Nephropathy', the term is now there (see screenshot below).

To Reproduce
Steps to reproduce the behavior:

1. Create a local knowledge base using the HPO profile
2. Go to the knowledge base page
3. Search for Nephropathy and select it
4. Click on New Subclass and create one called Ischemic Nephropathy
5. See the "Whoops..." page

Expected behavior
No error.

Screenshots

Caused by: java.lang.NullPointerException: Cannot invoke "de.tudarmstadt.ukp.inception.kb.model.KnowledgeBase.getClassIri()" because "this.kb" is null
	at de.tudarmstadt.ukp.inception.kb.querybuilder.SPARQLQueryBuilder.limitToClasses(SPARQLQueryBuilder.java:1962) ~[classes/:?]
	at de.tudarmstadt.ukp.inception.kb.querybuilder.SPARQLQueryBuilder.forClasses(SPARQLQueryBuilder.java:537) ~[classes/:?]
	at de.tudarmstadt.ukp.inception.kb.KnowledgeBaseServiceImpl.getParentConceptList(KnowledgeBaseServiceImpl.java:1177) ~[classes/:?]
	at de.tudarmstadt.ukp.inception.kb.KnowledgeBaseServiceImpl$$FastClassBySpringCGLIB$$77259834.invoke(<generated>) ~[classes/:?]
        ...

Please complete the following information:

• Version and build ID: 31.0