Accessing a page that doesn't exist causes a fatal error in EntityRouteHelper::getContentEntityFromRoute()

Problem/Motivation

That service does not check if there is a route object before accessing it. This is very likely caused by 📌 Use a placeholder for the navigation toolbar Active but there might be other cases where this could have happened as well.

Steps to reproduce

Visit /does-not-exist.

Note that the error is only appended to the regular response and not immediately visible because it's a big pipe lazy builder.

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet