Hi,
Thank you for you feedback. The problem is actually in the administrator/components/com_installer/src/Model/DatabaseModel.php file.
The code
$folderTmp = (string) $installationXML->update->schemas->schemapath[0];
assumes that any extension installed has either a sql/updates folder or a schemas node in the XML file of the extension.
This mijo_redirect plugin adds nothing to the database, so it doesn't requires a sql/updates folder nor a schemas node in the XML file.
The Joomla documentation on how to create a plugin for Joomla 4 also doesn't mention anything about this:
docs.joomla.org/J4.x:Creating_a_Plugin_for_Joomla
So it's actually the file administrator/components/com_installer/src/Model/DatabaseModel.php which needs to be modified.
The line
if ($installationXML !== null) {
just before the line of the notice should be changed to something like this :
if ($installationXML !== null && isset($installationXML->update->schemas->schemapath)) {
I would recommend reporting the issue on Joomla's github:
github.com/joomla/joomla-cms/issues