Jetty version(s)
12.0.13

Jetty Environment
Any

Java version/vendor (use: java -version)
Any

OS type/version
Any

Description
If an instance of a Handler.Wrapper without a child is allowed to start and begin handling requests, the error message is unclear.

java.lang.NullPointerException: Cannot invoke "org.eclipse.jetty.server.Request$Handler.handle(org.eclipse.jetty.server.Request, org.eclipse.jetty.server.Response, org.eclipse.jetty.util.Callback)" because "this._handler" is null

We should make the start of this kind of handler fail the start and throw a meaningful message that the Handler.Wrapper is missing a child.