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

Description of the bug

If you run a project on a Linux without which you see the following output

[INFO] --- vaadin-maven-plugin:18.0.0.alpha1:prepare-frontend (default) @ starter-wizard ---
[ERROR] Failed to execute the command '[which, node]'
java.io.IOException: Cannot run program "which": error=2, No such file or directory
    at java.lang.ProcessBuilder.start (ProcessBuilder.j

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