Drupal 8.x

I have migrated from D6 to D8 and used the migrate module to accomplish this.

Everything seemed to be okay until I clicked the "Comment" link in the admin section
(/admin/content/comment)
I get this error
Fatal error: Call to a member function url() on a non-object in /FULL PATH STRING REMOVED/core/modules/rdf/rdf.module on line 238

Hello, I'm a few years out of drupal.
I decided to have a bit of fun with drupal 8, seeing it as very promising, but need to re-grasp all the concepts that changed in all these years.

I need to build a catalog of products. It will not be a shop, just a directory of products.
I also want to do it with D8, as a learning project. So exclude D7 and exclude Commerce.

There will be multiple types of products. 30, maybe 50+, still not planned in detail.
Each product page will list that product's attributes/detailed specifications.

Apart from the "universal" specifications like size (length, W, H, weight etc) there will be some very specific fields (sometime a few, sometime dozens very specific) different for every product type .

How do I load my Main menu in my Foundation Off canvas region off-canvas position-left?

In menu--main.html.twig I have below structure for my desktop menu:

I am seeking path of least resistance to quickly create a site and not be burdened by technical details and to move on quickly into using it for my purpose.

It will be a personal site for a side-gig. I originally intended to use ZF2 just because I know it and I know what I'll be in for and how long it will take me to make a site. But then chose to look into Drupal because it is a powerful CMS engine and can make sites fast as I understand.

I am totally totally new to Drupal I know next to nothing and I have never used a CMS before.

Hy

I can not insert a picture in the header background image. Why? Use Xampp, mayo-8x-1.2 and drupal8. Everything else works great.
I tried it with .png, .jpg and .gif. The biggest was 702KB.
Nothing happened, except that the logo disappears.

Hi,

I'm using Drupal 8's migrate system to import data from an XML source which has this simple layout: