Unhandled Exception / List index out of range error pressing bumper keys on Freedom Scientific Focus 80 Blue Braille display #8849

Steps to reproduce:

(Issue reported by a client via email. Log attached)
nvda (9).log

1. Open notepad with text which stretches over a line.
2. Press the right bumper bar down to scroll forward through text.

Actual behavior:

Exception generated, front panel keys on display stop working.
Additional text not read.

Expected behavior:

Display should scroll to read additional text.

System configuration:

Windows 7, 32 Bit with SP1.

NVDA Installed/portable/running from source:

NVDA 2018.3.2 installed. No add-ons, and using the Freedom Scientific display driver recommended in the NVDA User Guide: https://www.nvaccess.org/files/nvda/documentation/userGuide.html#FreedomScientificFocus

Does the issue still occur after restarting your PC?

Yes

Have you tried any other versions of NVDA?

Yes, this issue has been present "for several years".