Vaadin Framework version: 8, all versions

There is a major error in the class com.vaadin.server.VaadinSession (and possibly other classes too, if this is some kind of cargo cult programming in the Vaadin team)

Most public methods in the class use assertions to "check" if the session has the lock. This is fundamentally wrong. Assertions are the wrong tool to check preconditions in public meth

Since #8506 has been fixed, a Registration is supposed to be idempotent (or idempotent-ish at least), i.e. calling remove() multiple times has the same effect as calling it once.

However, this is not mentioned explicitely in the documentation:

https://github.com/vaadin/flow/blob/c009afc5c24d8a3d6abc14f6f5cd3faea80c1932/

Describe the bug
Running a simple, valid program in the JS.do example website may lead to a valid script terminating.

Error message is:
`Operation """": no method found, argument in overloaded package WebPerl::JSObject.
exit(255) called, but NO_EXIT_RUNTIME is set, so halting execution but not exiting the runtime or preventing further async execution (build with NO_EXIT_RUNTIME=0, if you