Right clicking on a blank area in the tags listing causes a crash.

Steps to reproduce:

1. Click "add folder"

2. Select a folder with some tracks

3. Click on any of the just loaded tracks

4. In the bottom area where the tags are listed, right click on the blank area after the last tag (may need to make window taller to make this area appear)

5. Receive traceback:

Traceback (most recent call last):
File "/usr/lib/python3.6/site-packages/picard/ui/metadatabox.py", line 292, in contextMenuEvent
column = item.column()
AttributeError: 'NoneType' object has no attribute 'column'