[Bug]: If devices in DeviceManager all have a width specificed an editor breaking error occurs

If all devices specified for the DeviceManager have a width set, an editor breaking error occurs: "TypeError: contRules is null". I'm seeing this in my local project with v0.14.52.

jsfiddle: https://jsfiddle.net/5ycmoftj/1/

I think what is happening is that CssRulesView.js render() is creating div fragments as containers for different media targets, based on the widths of the devices in the device manager. If you don't have a device set up with a undefined width, there is no place for css rules that don't specify a media width to go. It doesn't create a container for them.