LivyClient comes out NullPointException while return spark-sql result

class SQLQueryJob(tableName: String, sql: String, schemas: JMap[String,String]) extends Job[JList[Row]] {

  override def call(jc: JobContext): JList[Row] = {
    val session:SparkSession = jc.sparkSession()
    implicit val hConfig = HbConfig()
    query(session, tableName, sql, schemasToStructType(schemas)).collectAsList()
  }

  private[this] def query(sparkSession: SparkSession,
                          tableName: String,
                          sql: String,
                          schemas: StructType)
                         (implicit config: HbConfig): DataFrame = {
    val rdd = sparkSession.readHbase(tableName, COLUMN_FAMILY, schemas)
    val df = sparkSession.createDataFrame(rdd, schemas)
    df.createOrReplaceTempView(tableName)
    sparkSession.sql(sql)
  }
}

List<Row> rows = livyClient.submit(job).get();