Got the following when try to save Website Setting->Appearance (Keep loading) [Wed Mar 04 10:14:22.214321 2020] [php7:notice] [pid 2168:tid 1888] [client ::1:50528] PHP Notice: Trying to access array offset on value of type null in C:\\xampp\\htdocs\\tigadua\\plugins\\themes\\default\\DefaultThemePlugin.inc.php on line 204, referer: http://localhost/tigadua/index.php/dia/management/settings/website
It depends on your install strategy. Did you use the pre-compiled packages or did you use git clone ... or git pull ... ?
Probably, the above changes are only deployable for the next few days if you use the codebase via git and recompile javascript via npm run build, as described on GitHub - pkp/ojs: Open Journal Systems