TypeError when calling getSQL with invalid rules

[eagano]

The getSQL() function is throwing a TypeError when there is at least one invalid rule present.

Steps to reproduce, from the QueryBuilder demo page (http://querybuilder.js.org/demo.html):

1. scroll down to Import/Export demo
2. open the console
3. click Reset
4. click SQL (any of the four SQL choices)
5. TypeError thrown on null group (argument to parse() in getSQL)

Alternatively:

1. Click Set rules from SQL
2. Click Add Rule
3. Click SQL (any of the four SQL choices)
4. TypeError thrown on null group (argument to parse() in getSQL)

It appears that validate() returns null when there are rules in error.