Callback error when changing merchant plant analysis period

[brtietz]

Describe the bug

When changing the analysis period in PVWatts-Merchant plant, I get the following error:

Could not evaluate callback function:analysis_period->on_change
[61]: reference to unassigned variable:change_curtailment_price_length
[61]: error: _s
[54]: eval error in statement list

To Reproduce

Steps to reproduce the behavior:

1. Open a merchant plant case
2. Go to financial assumptions
3. Change the analysis period
4. See error

Expected behavior

change_curtailment_price_length should be updated to remove this error (haven't dug deeply enough to describe this better)

Operating System and Version

• Version: Appears to just be on develop, 2025.4.16 does not display this behavior

[brtietz]

Upon further testing, this also appears in cashloan. Will update the PR to be more robust and fully consider all financial models.