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

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;
+                }
             }
         }