Bugzilla 448776 : WEIGHTEDAVE Expression is not set correctly

Original description of problem:
The Expression Field in a weighted average aggregation, does not resolve
to row[xxxxx] like it should, rather just xxxxx.

The workaround is to go through the fx expression builder).

The steps to recreate bug:
Create a data source and data set.
Create a table with a group.
From palette quick tools, drag "Aggregation" to the table.
Choose function is "WEIGHTEDAVE"
Try to populate "Expression" field by choosing data set column from the drop down list.
Note that the field is populated with <column name>.
It should be row[<column name>]