As you know this is the beta program and I have a development environment set up like described in the wiki.

Every time I start the progam I get this message:

can't invoke "event" command: application has been destroyed while executing "event generate $w <<ThemeChanged>>" (procedure "ttk::ThemeChanged" line 6) invoked from within "ttk::ThemeChanged" 

@deajan some information requested by you as per #175 (comment)
OS = W11 Enterprise
python = Python 3.12.2
bitness (?) = x64-based pc
npbackup version = 2026-03-17 21:34:18,422 :: INFO :: GUI: npbackup 3.1.0-beta2-windows-gui-x64-public-3.12-i 2026031603

It does not matter if I start with the variables set for dark theme and scaling:

set NPBACKUP_DPI_AWARENESS=true set NPBACKUP_SCALING=1.0 set NPBACKUP_THEME=dark 

...or with no variables at all and a light theme.

when I load it with a dummy config file i get this:

2026-03-17 22:14:17,755 :: INFO :: Loaded config 6FB00A28 in C:\_GIT\npbackup\npbackup.conf can't invoke "event" command: application has been destroyed while executing "event generate $w <<ThemeChanged>>" (procedure "ttk::ThemeChanged" line 6) invoked from within "ttk::ThemeChanged" 2026-03-17 22:14:18,084 :: ERROR :: Repo cannot be empty 

which suggests the error is thrown between loading the config file and processing the config file? Or is this multi-threaded.... In case of an existing config npbackup only draws ONE window, the main window.

In all the files of the 'development' folder from git there is only one file with any mention of ThemeChanged :
"C:_GIT\npbackup\venv\Lib\site-packages\sv_ttk\sv.tcl"

Alright, let me now if there is anything else you need.