Missing plugin (does not exist) error when choosing a blb layout as default laout

Patch for testing

Patch works and applies. It fixes the error for me. I still got an error on that page but that was from before the patch: it had saved the invalid layout id (blb_col_1) as default layout, so when opening the set default section layout page, it tries to load that invalid plugin id / layout so it gives an error.

Fixed it by exporting my config (drush cex -y) and then adjusting the default_section inside config/sync/core.entity_view_display.node.MYBUNDLE.default.yml from:

    default_section:
      layout_plugin: blb_col_1

to:

    default_section:
      layout_plugin: bootstrap_layout_builder:blb_col_1

and then importing the updated config (drush cim -y).
Now that the default layout is a valid one, I could open the set default section layout page without any errors, and I could change the default layout into any existing layout and save it without errors.