Exception thrown when using MySQL 5.7 in strict mode

When using the widget with MySQL 5.7 or later in strict mode, the following exception is raised:

General error: 3065 Expression #1 of ORDER BY clause is not in SELECT list

This is because in the function _term_reference_tree_get_children() there is an SQL query created with order by t.weight, but t.weight is not present in the list of fields.