When a context doesn't exist a new context is created during lookup. This context is placed in a MAP with the method putIfAbsent, which returns the previous value in the map (in this case null because the context didn't exist). This previous value is assigned to the local variable and is returned. --> Method returns null.

fix: don't assign the previous value in the map, but keep and return the new created context (org.apache.jmeter.protocol.jms.client.InitialContextFactory.lookupContext: 74)