Made a bit more robust.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@36962 71c3de6d-444a-0410-be80-ed276b4c234a

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

@@ -1724,7 +1724,10 @@ public abstract class Run <JobT extends Job<JobT,RunT>,RunT extends Run<JobT,Run
      * @since 1.305
      */
     public EnvVars getEnvironment(TaskListener log) throws IOException, InterruptedException {
-        EnvVars env = Computer.currentComputer().getEnvironment().overrideAll(getCharacteristicEnvVars());
+        EnvVars env = getCharacteristicEnvVars();
+        Computer c = Computer.currentComputer();
+        if (c!=null)
+            env = c.getEnvironment().overrideAll(env);
         String rootUrl = Hudson.getInstance().getRootUrl();
         if(rootUrl!=null) {
             env.put("HUDSON_URL", rootUrl);