Duplicate dictionary AssertionError has no clear user facing message

[didoesdigital]

Describe the bug

When the plover.cfg file contains duplicate dictionaries in the dictionaries array, Plover shows…

• Qt GUI error, AssertionError
• Default dictionaries as "loading", even if those files aren't there
• No sign of active custom dictionaries, even though some are working

To Reproduce

Steps to reproduce the behavior:

1. Add duplicate dictionaries to plover.cfg e.g.:

dictionaries = [{"enabled": true, "path": "~/dictionaries/css-alignment.json"}, {"enabled": true, "path": "~/dictionaries/css-alignment.json"}]

2. Open Plover
3. See error

plover.log file shows:

2021-12-29 20:55:23,350 [MainThread] ERROR: Qt GUI error
Traceback (most recent call last):
  File "/Users/runner/work/plover/plover/build/osxapp/Plover.app/Contents/Frameworks/Python.framework/Versions/Current/lib/python3.9/site-packages/plover/gui_qt/dictionaries_widget.py", line 225, in _on_config_changed
  File "/Users/runner/work/plover/plover/build/osxapp/Plover.app/Contents/Frameworks/Python.framework/Versions/Current/lib/python3.9/site-packages/plover/gui_qt/dictionaries_widget.py", line 188, in _reset_items
AssertionError

Here's a link to the error code: https://github.com/openstenoproject/plover/blob/master/plover/gui_qt/dictionaries_widget.py#L188

Expected behavior

A more user-friendly error message, such as "Duplicate dictionaries. Check plover.cfg for duplicate dictionaries."

Operating system

• OS: macOS Catalina
• Plover 4.0.0.dev10+120.g4394ef1

Hardware

N/A