Handle undefined id on updateIframeSrc

## Before

![screenshot 2014-12-01 11 34 02](https://cloud.githubusercontent.com/assets/1151489/5258341/fb94a996-79b1-11e4-966c-299634d39a6d.png)

## After

![screenshot 2014-12-01 23 27 32](https://cloud.githubusercontent.com/assets/1151489/5258347/04e105f8-79b2-11e4-9d84-ad83db2edd80.png)