In line 84 of eznotificationtransport.php there is an check if variable $impl === null.
But this variable will only be declared before if an other statement is true ( if ( $fetchInstance ) ).

So add to line 49 a base declaration to prevent errors:

static function instance( $transport = false, $forceNewInstance = false )
    {
->    $impl = null;
        $ini = eZINI::instance( 'notification.ini' );