Minor bug fixed in the Merge Dialogue

[N-thony]

Fixes #7414
@africanmathsinitiative/developers this is ready for review.

[lloyddewit]

@N-thony Thank you for this PR.
I see that you have renamed some variables but I can't see what else has changed.
Which lines fix the bug?
Thanks

[rdstern]

@N-thony I assume this is the main prepare > reshape > merge dialogue, but I'm not sure what to do to check this is resolved? What data and example gace that error before?

[N-thony]

@N-thony I assume this is the main prepare > reshape > merge dialogue, but I'm not sure what to do to check this is resolved? What data and example gace that error before?

@rdstern while testing PR #7404 with the survey dataset, tried to open the Merge dialogue, you will get the bug that this PR attends to fix. To quickly avoid that bug in the merge version since it was easy to fix, I did the fix in this PR.
First test PR #7404 and in that same branch try to open Merge dialogue and you will get the bug.

[rdstern]

No problem with Merge here, which there was with the pull request @N-thony pointed me to. I hope all will still work when both are merged.
Ok from my side now. Over to @lloyddewit or @ChrisMarsh82

[lloyddewit]

@N-thony Thank you for this PR. I see that you have renamed some variables but I can't see what else has changed. Which lines fix the bug? Thanks

I don't see any functional changes in the code, so I can't see how this PR has fixed a bug.
Am I missing something?

[lloyddewit]

The functional change in this PR is that 'New' has been added to lines 22, 23, and 29.
This is a temporary fix for issue #7414 and part of #7438.
The real issue is that an event handler in this dialog is called before SetDefaults(). This issue was introduced by PR #7404.
Another complication is that this PR was branched from master before PR #7404 was merged so we cannot fully test this branch.
@N-thony and I discussed. On balance, we decided to merge this PR and address the root cause of the problem in issue #7438.