Fix a bug occurred after closing one of the copied tabs

Atom seems to be a stranger after closing a tab which is opened in multiple panes.

In the following GIF, each time I pressed enter key, the next line seems to be lost and a error log appeared in the console. Once I reopen the tab, it works correctly.

![autocomplete-plus-bug](https://cloud.githubusercontent.com/assets/551939/2797657/8d050e12-cc37-11e3-919a-bd28475d48eb.gif)