Order does not work

Problem/Motivation

Currently only the initial ordering works. A later ordering by clicking the column header fails.

Steps to reproduce

Create a view with a sortable field and try to sort by it.

Proposed resolution

The problem is this line of code. When $field is NULL the call of CMRFViewsFieldNameUtil::normalize() fails because it requires a string and doesn't accept NULL.

This line should be:

$alias = '' === $alias ? $field : $this->getFieldByAlias($alias);