"PHP Warning: get_class() expects parameter 1 to be object, null given in /usr/share/webapps/mediawiki/extensions/OpenID/SpecialOpenIDLogin.body.php on line 154"
http://php.net/manual/en/function.get-class.php
"7.2.0 NULL was removed as the default value for object, and is no longer a valid input."
I think this fixes it:
...
$class = is_null( $wgOpenIDForcedProvider ) ? get_class() : get_class($wgOpenIDForcedProvider);
if ( $class == 'OpenIDProvider' ) {
$inputFormHTML .= $wgOpenIDForcedProvider->getLoginFormHTML();
...