fix: createPath failed for offline Slaves

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@14796 71c3de6d-444a-0410-be80-ed276b4c234a

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

@@ -288,7 +288,9 @@ public abstract class Slave implements Node, Serializable {
     }
 
     public FilePath createPath(String absolutePath) {
-        VirtualChannel ch = getComputer().getChannel();
+        SlaveComputer computer = getComputer();
+        if (computer==null) return null; // offline
+        VirtualChannel ch = computer.getChannel();
         if(ch==null)    return null;    // offline
         return new FilePath(ch,absolutePath);
     }