How to use CellProfiler Plate Viewer?

I’m trying to use CellProfiler’s Plate Viewer, but when I click on Data Tools >> Plate Viewer nothing happens. I’m using a dataset where I’ve extracted a metadata field called “Well” from the filenames.

I’m running CP 3.1.8, and I’ve tried this on a Mac and PC with the same result.

What am I missing?

Edit: below is the error that gets printed to the log when using CP on Windows:

Traceback (most recent call last):
  File "CellProfiler\cellprofiler\gui\", line 1205, in __on_plateviewer
  File "CellProfiler\cellprofiler\gui\", line 239, in __init__
  File "CellProfiler\cellprofiler\gui\", line 308, in on_update
  File "site-packages\wx-3.0-msw\wx\", line 13033, in SetItems
  File "site-packages\wx-3.0-msw\wx\", line 12870, in AppendItems
TypeError: String or Unicode type required