copying Better BibTex citekey

While attempting to copy a Better BibTex citekey an error is raised. I have set COPY_CITEKEY_MOD differently (fn, cmd, ctrl) to check, but find the same result.

This is the error from the log:

```13:11:27 util.py:273 INFO [0.02s] load Better Bibtex data
13:11:27 workflow.py:2080 ERROR 'NoneType' object has no attribute 'tags'
Traceback (most recent call last):
File "./lib/workflow/workflow.py", line 2073, in run
func(self)
File "./zh", line 714, in main
return do_reindex()
File "./zh", line 597, in do_reindex
app.update_index(force=False)
File "./lib/zothero/core.py", line 166, in update_index
self.index.update(self.zotero, force)
File "./lib/zothero/index.py", line 277, in update
if not self._update(zot, force):
File "./lib/zothero/index.py", line 327, in _update
tags = u' '.join(e.tags)
```

The Alfred window shows "Open in Zotero" when using the chosen key.