Not a real defect since IsUnix command does not return null. But it still makes sense to fix that.

public Launcher createLauncher(TaskListener listener) throws IOException, InterruptedException {
        if(channel==null)
            return new LocalLauncher(listener);
        else
            return new RemoteLauncher(listener,channel,channel.call(new IsUnix()));
    }

    private static final class IsUnix extends MasterToSlaveCallable<Boolean,IOException> {
        public Boolean call() throws IOException {
            return File.pathSeparatorChar==':';
        }
        private static final long serialVersionUID = 1L;
    }

We hit it in another logic branch. I will fix it

https://github.com/jenkinsci/jenkins/pull/2923

Code changed in jenkins
User: Oleg Nenashev
Path:
core/src/main/java/hudson/model/Slave.java
http://jenkins-ci.org/commit/jenkins/78a42d5a4a5d545324c2d3230de6947e1ec8806e
Log:
JENKINS-38527 - Prevent NullPointerException in Slave#createLauncher() and add cause diagnostics (#2923)

• JENKINS-38527 - Prevent NullPointerException in Slave#createLauncher() and add cause diagnostics

The original issue comes from the isUnix() unboxing, but we can also get into an issue later if we pass a null Channel instance to the logic.
This change adds some diagnostics which discovers potential root causes of such potential NPEs due to the race conditions with Computer reconnection

• JENKINS-38527 - Also handle cases when Channel#isClosingOrClosed() as @stephenc suggested

The pull request has been merged towards 2.68. Marking as LTS candidate

Code changed in jenkins
User: Oleg Nenashev
Path:
core/src/main/java/hudson/model/Slave.java
http://jenkins-ci.org/commit/jenkins/c2163c5b3cb4a0c42cdc31cf404a82bcecb05082
Log:
JENKINS-38527 - Prevent NullPointerException in Slave#createLauncher() and add cause diagnostics (#2923)

• JENKINS-38527 - Prevent NullPointerException in Slave#createLauncher() and add cause diagnostics

The original issue comes from the isUnix() unboxing, but we can also get into an issue later if we pass a null Channel instance to the logic.
This change adds some diagnostics which discovers potential root causes of such potential NPEs due to the race conditions with Computer reconnection

• JENKINS-38527 - Also handle cases when Channel#isClosingOrClosed() as @stephenc suggested

(cherry picked from commit 78a42d5a4a5d545324c2d3230de6947e1ec8806e)