interrupt “color-picker:open” command if editor is unavailable

Fixed a bug causing “Uncaught TypeError: Cannot read property 'getCursorBufferPosition' of undefined” error appears when accidentally calling color-picker from screen with no editor available, for example, settings screen (see attached screenshot).
![screenshot 2014-11-16 20 47 08](https://cloud.githubusercontent.com/assets/742056/5061646/0ee19cca-6dd2-11e4-89a7-5f3a4deecbfc.png)