[FIXED JENKINS-21999] If a slave node does not exist it will throw a

null pointer exception. Instead we create a dummy launcher and return
that if it is unable to get the slave node.

core/src/main/java/hudson/Launcher.java

@@ -841,6 +841,31 @@ public abstract class Launcher {
         }
     }
 
+    public static class DummyLauncher extends Launcher {
+
+        public DummyLauncher(TaskListener listener) {
+            super(listener, null);
+        }
+
+        @Override
+        public Proc launch(ProcStarter starter) throws IOException {
+            listener.error("Can not call launch on a dummy launcher.");
+            return null;
+        }
+
+        @Override
+        public Channel launchChannel(String[] cmd, OutputStream out, FilePath workDir, Map<String, String> envVars) throws IOException, InterruptedException {
+            listener.error("Can not call launchChannel on a dummy launcher.");
+            return null;
+        }
+
+        @Override
+        public void kill(Map<String, String> modelEnvVars) throws IOException, InterruptedException {
+            // Kill method should do nothing.
+        }
+    }
+
+
     /**
      * Launches processes remotely by using the given channel.
      */

core/src/main/java/hudson/model/Slave.java

@@ -343,9 +343,21 @@ public abstract class Slave extends Node implements Serializable {
 
     }
 
+    /**
+     * Creates a launcher for the slave.
+     *
+     * @return
+     *      If there is no computer it will return a {@link hudson.Launcher.DummyLauncher}, otherwise it
+     *      will return a {@link hudson.Launcher.RemoteLauncher} instead.
+     */
     public Launcher createLauncher(TaskListener listener) {
         SlaveComputer c = getComputer();
-        return new RemoteLauncher(listener, c.getChannel(), c.isUnix()).decorateFor(this);
+        if (c == null) {
+            listener.error("Issue with creating launcher for slave " + name + ".");
+            return new Launcher.DummyLauncher(listener);
+        } else {
+            return new RemoteLauncher(listener, c.getChannel(), c.isUnix()).decorateFor(this);
+        }
     }
 
     /**