Fix JavaScript error on expanding MDX editor pane.
Fix JavaScript error on expanding MDX editor pane.

(Regression from #68 : f42e3f1e07f756192636ae6db4b085aa03815af5)
mysticfall/pivot4j,seddikouiss/pivo4j,seddikouiss/pivo4j,mysticfall/pivot4j,seddikouiss/pivo4j,mysticfall/pivot4j