2529-More corrections for add layer and the config API

[ychoquet]

Description

QA done on hte add layer and the config API revealed some issues. This PR proposes corrections to be applied to the code.

Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Using the add-layer.html template.

Deploy URL: https://ychoquet.github.io/GeoView

Checklist:

• I have build (rush build) and deploy (rush host) my PR
• I have connected the issues(s) to this PR
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have created new issue(s) related to the outcome of this PR is needed
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

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This change is 

[jolevesq]

Reviewed 18 of 19 files at r1, all commit messages.
Reviewable status: 18 of 19 files reviewed, all discussions resolved

[jolevesq]

Reviewed 19 of 19 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @ychoquet)

[jolevesq]

Reviewed 4 of 4 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @ychoquet)