The error comes from ext.cx.tools.categories.js line line 498.
You can test it with any article from https://es.wikipedia.org/wiki/Especial:P%C3%A1ginasSinCategorizar in the Spanish Wikipedia — for example, 'Andrew Wylie' (check that nobody added categories before you load it.)
There's a question of whether the screen should show "0 categories" in both columns or just nothing.