When I started the theme-adjuster, I had opened a theme with no theme-parameters at all.
When I switched to the default v6-theme, the theme-adjuster showed me this error-message:
Code:
...PI_4.1.001\Scripts\Cockos\Default_6.0_theme_adjuster.lua:863: bad argument #1 to 'ThemeLayout_SetParameter' (number expected, got nil)
It happens as well, when you use the default v5-Theme, run the theme-adjuster and then switch over to v6.
I've tested it under Reaper 6.11/x64.