Opening drop down list problem

[Gennady77]

When theme of ui-select is default (bootstrap) and search-enabled="false" then there is an exception by clicking on arrow down button. Drop down is not shown in this case. I corrected plunker sample. This defect reproduced on "Selectize theme" field.

http://next.plnkr.co/edit/H0Uzd8io1uvyphPY

Angular: 1.5.11

UI-Select: 0.19.8

Bootstrap: 3.3.7