Step 1 in fixing 1859: this stops the crashing, but virtual fields are not exported
step 2 of fixing export from grids when there are virtual fields. Note export hidden columns includes all table fields and any virtual fields which are displayed, but it does not pick up virtual fields which are not displayed.
when exporting hidden columns, pick up all virtual fields, even those not on display.
fixed comment
Merge pull request #371 from timrichardson/issues/1859

Issues/1859 export from grid fails when virtual fields are present