12570 - Fix Variant Unit field is Out of Sync with the Pop-out

[chahmedejaz]

What? Why?

• Closes #12570
• We are creating a new variant object and passing it to the new variant row.
• This variant is empty and hence has no unit_value present in it.
• This PR creates and uses the helper to build this new variant object such that it has the unit_value of the previous variant,

What should we test?

• The expected behavior mentioned in the issue should meet

Changelog Category (reviewers may add a label for the release notes):

• [ ] User facing changes
• [ ] API changes (V0, V1, DFC or Webhook)
• [x] Technical changes only
• [ ] Feature toggled