Changes to Functions Affecting All Domains in Fit Script Generator Apply to Incorrect Functions

[cailafinn]

Describe the bug
Found during manual testing for 6.11. Not a regression.

Changing a value in one of the functions that have been applied to all domains tries to update the others, but if another function has additional functions (like a background) that take up the same index as the changed value, then it attempts to edit the wrong function.

To Reproduce

• Follow the instructions here: https://docs.mantidproject.org/nightly/interfaces/general/Fit%20Script%20Generator.html#usage-example-for-simultaneous-fitting

Expected behavior
The same function should be updated in all domains (or none of them, the middle ground solution is much more confusing).

Platform/Version (please complete the following information):

• OS: All.

Additional context