[FIXED JENKINS-3265] Hotfix for regression in matrix project loading from...

[FIXED JENKINS-3265] Hotfix for regression in matrix project loading from 25084f52.(cherry picked from commit 3370e6ed)

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

@@ -666,6 +666,9 @@ public class MatrixProject extends AbstractProject<MatrixProject,MatrixBuild> im
     }
 
     public MatrixConfiguration getItem(Combination c) {
+        if (configurations == null) {
+            return null;
+        }
         return configurations.get(c);
     }