so good! :)
so you deleted the theme which was set before : upload it
what was this theme
hello did you delete all files before the upgrade ?
Remove all files of your current Piwigo installation, with the following exceptions, do NOT delete these directories:
hello did you delete all files before the upgrade ? because the theme is missing the theme
For dev : We should also do something if themeconf.inc.php can't be opened, like using the default theme
Piwigo 2.3.beta2 => RC2