Fix Fatal error on getDIType() for SMW_QP_Category

Needs https://gerrit.wikimedia.org/r/#/c/132815/

Reported by [0]

[0] http://wikimedia.7.x6.nabble.com/Odd-error-on-ask-tp5027423.html
Merge pull request #312 from SemanticMediaWiki/bug-qp-category

Fix Fatal error on getDIType() for SMW_QP_Category