I propose this simple fix.

web.php, Line 520

NOW

// We just need the prefix since we have a path relative to the root.	
if ($url[0] == '/')
{
    $url = $prefix . $url;
}

AFTER

// We just need the prefix since we have a path relative to the root.
if( isset($url[0])  && ($url[0] == '/') )
{
	$url = $prefix . $url;
}

Are you ok for a pull request ? (See code Above)

The fact that an error is thrown there
does not mean that the code has some bug
(unless you can prove that a valid usage of the API is made)

Possibly (Artio) JoomSEF is overriding JUri class or something similar

Try using latest version of the extension, if you are doing this already,

• then report this issue to the author of the extension

Also possibly this notice can be safely ignored,
-- i say the above since the author of the extension has not made a fix for it
and since author claims extension is compatible with J3.7.5

so can try to set error_reporting in Joomla global configuration to "system default" (it is recommeded not to print notices/warnings/errors in live sites)

@ ggppdk

Thx, i've already reported the issue to the owner.

Set to "closed" on behalf of @franz-wohlkoenig by The JTracker Application at issues.joomla.org/joomla-cms/17959

closed as no Core-Issue.