#vulnerable code 
                              public static String getCluster(Configuration jobConf) {
    String jobtracker = jobConf.get(JOBTRACKER_KEY);
    if (jobtracker == null) {
      jobtracker = jobConf.get(RESOURCE_MANAGER_KEY);
    }
    // strip any port number
    int portIdx = jobtracker.indexOf(':');
    if (portIdx > -1) {
      jobtracker = jobtracker.substring(0, portIdx);
    }
    // An ExceptionInInitializerError may be thrown to indicate that an exception occurred during
    // evaluation of Cluster class' static initialization
    String cluster = Cluster.getIdentifier(jobtracker);
    return cluster != null ? cluster: null;
  } 
                            #location 7 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                              public static String getCluster(Configuration jobConf) {
    String jobtracker = jobConf.get(JOBTRACKER_KEY);
    if (jobtracker == null) {
      jobtracker = jobConf.get(RESOURCE_MANAGER_KEY);
    }
    String cluster = null;
    if (jobtracker != null) {
      // strip any port number
      int portIdx = jobtracker.indexOf(':');
      if (portIdx > -1) {
        jobtracker = jobtracker.substring(0, portIdx);
      }
      // An ExceptionInInitializerError may be thrown to indicate that an exception occurred during
      // evaluation of Cluster class' static initialization
      cluster = Cluster.getIdentifier(jobtracker);
    }
    return cluster;
  }