[FIXED HUDSON-3696] Contract violation. This shouldn't be null.

changelog.html

@@ -46,6 +46,9 @@ Upcoming changes</a>
   <li class=bug>
     Fixed a pipe leak to child processes.
     (<a href="http://issues.hudson-ci.org/browse/HUDSON-8244">issue 8244</a>)
+  <li class=bug>
+    Fixed an NPE in ComputerRetentionWork
+    (<a href="http://issues.hudson-ci.org/browse/HUDSON-3696">issue 3696</a>)
   <li class=rfe>
     Debian package init script now honors <tt>~/.profile</tt>.
   <li class=rfe>

core/src/main/java/hudson/slaves/SlaveComputer.java

@@ -445,7 +445,7 @@ public class SlaveComputer extends Computer {
 
     public RetentionStrategy getRetentionStrategy() {
         Slave n = getNode();
-        return n==null ? null : n.getRetentionStrategy();
+        return n==null ? RetentionStrategy.INSTANCE : n.getRetentionStrategy();
     }
 
     /**