Error when collapsing a mixed group

[thinkh]

Steps to reproduce

1. Stratify a categorical column
2. Switch to overview
3. Select expand region with first row (to get the collapse arrow)
4. Try to collapse the group

Observed behavior

Uncaught TypeError: Cannot read property '1' of undefined
    at LineUpJS.js:62
    at Array.forEach (<anonymous>)
    at LineUpJS.js:62
    at Array.forEach (<anonymous>)
    at t.renderColumns (LineUpJS.js:62)
    at t.update (LineUpJS.js:62)
    at e.update (LineUpJS.js:62)
    at Object.<anonymous> (LineUpJS.js:62)
    at o.apply (LineUpJS.js:62)
    at e.a (LineUpJS.js:43)

Expected behavior

• No error when collapsing