[FIXED JENKINS-17728] ParentBuildAction.parent can sometimes be null.

changelog.html

@@ -60,6 +60,9 @@ Upcoming changes</a>
   <li class=bug>
     Third-party license display for core was broken since 1.506.
     (<a href="https://issues.jenkins-ci.org/browse/JENKINS-17724">issue 17724</a>)
+  <li class=bug>
+    NPE from <code>MatrixConfiguration.newBuild</code>.
+    (<a href="https://issues.jenkins-ci.org/browse/JENKINS-17728">issue 17728</a>)
   <li class=rfe>
     <code>/about</code> now links to license information for plugins as well.
   <li class=rfe>

core/src/main/java/hudson/matrix/MatrixConfiguration.java

@@ -232,7 +232,10 @@ public class MatrixConfiguration extends Project<MatrixConfiguration,MatrixRun>
         MatrixBuild lb = getParent().getLastBuild();
         for (Action a : actions) {
             if (a instanceof ParentBuildAction) {
-                lb = ((ParentBuildAction) a).parent;
+                MatrixBuild _lb = ((ParentBuildAction) a).parent;
+                if (_lb != null) {
+                    lb = _lb;
+                }
             }
         }