Fixed usage of category selector with new category base class

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #740, zikula-modules/KnowledgeBase#8
| Refs tickets  | 
| License       | MIT
| Doc PR        |