[BUG] Simple GroupBy throws NPE

[adamretter]

The following query throws a NullPointerException:

let $x := (10, 20, 30)
group by $y := $x < 30
return
  $x

The stack trace is:

java.lang.NullPointerException: Cannot read field "initialized" because "data" is null
        at org.exist.xquery.GroupByClause.eval(GroupByClause.java:100) ~[exist-core-7.0.0-SNAPSHOT.jar:7.0.0-SNAPSHOT]
        at org.exist.xquery.LetExpr.eval(LetExpr.java:110) ~[exist-core-7.0.0-SNAPSHOT.jar:7.0.0-SNAPSHOT]
        at org.exist.xquery.AbstractExpression.eval(AbstractExpression.java:71) ~[exist-core-7.0.0-SNAPSHOT.jar:7.0.0-S
NAPSHOT]
        at org.exist.xquery.PathExpr.eval(PathExpr.java:280) ~[exist-core-7.0.0-SNAPSHOT.jar:7.0.0-SNAPSHOT]
        at org.exist.xquery.AbstractExpression.eval(AbstractExpression.java:71) ~[exist-core-7.0.0-SNAPSHOT.jar:7.0.0-S
NAPSHOT]
        at org.exist.xquery.XQuery.execute(XQuery.java:445) ~[exist-core-7.0.0-SNAPSHOT.jar:7.0.0-SNAPSHOT]
        at org.exist.xquery.XQuery.execute(XQuery.java:348) ~[exist-core-7.0.0-SNAPSHOT.jar:7.0.0-SNAPSHOT]
        at org.exist.xquery.XQuery.execute(XQuery.java:335) ~[exist-core-7.0.0-SNAPSHOT.jar:7.0.0-SNAPSHOT]

Tested on eXist-db 7.0.0.-SNAPSHOT