Fixed the 'Edit Plugin' context menu option from breaking

For some reason, CKEditor won't display a context menu when asked, instead giving the following error:

plugin.js:140 - Uncaught TypeError: Cannot call method 'indexOf' of null

It seems the code tries to decide whether or not the currently selected element is indeed a CMSPlugin by checking it's id for the string 'plugin_obj', and if it isn't, it hides the context menu item.

Only for whatever reason, element.getAttribute('id') for me returns 'null'. An alternative method element.$.id seems to work fine.

This is on Chromium and Firefox on Ubuntu 13.10, using the latest version of everything.