Fix bug in two sample non-parametric dialog.

Commit fc6620099bbc53762095569d5356b67a81a85402 changed all dialog
boxes such that the previous (existing) dialog widget and all
children are re-used each time the dialog is summoned, instead of
creating a new one each time.

The two sample non-parametric test, however has some special code
on activation to populate the dialog, and this had not been moved
to the conditional case - hence the pertinent widgets were getting
added each time.  This resulted in the dialog being overpopulated.

This change fixes these things.