Fix undefined index, fix form headings

[Hlavtox]

Questions	Answers
Description?	Read below
Type?	bug fix / refacto
BC breaks?	no
Deprecations?	no
Fixed ticket?	Fixes PrestaShop/PrestaShop#30221
How to test?	Generate sitemap and see that no notices are thrown and the config form looks somehow OK.

Changes

• Error about directory not being writeable is shown by notification, not generated manually. I used context error notifications for this.
• Fixed panel headings by using proper syntax. It should be a div panel-heading, not a h3. Fixes a tiny bug.
  
• Fixed notices where it was trying to access [0] on a boolean. Simmilar fix was applied for products, but not for categories.

[AureRita]

Hi @Hlavtox

Thank you for your PR, I tested it on 1.7.8.7, 8.0.X and develop and it works, as we can see here :

Untitled_.Dec.7.2022.3_00.PM.webm

[kpodemski]

Thanks @Hlavtox :)