an error occured when write parquet files to disk. any advise? I want to know the reason.thanks ``` 16/03/29 18:31:48 WARN hadoop.ParquetOutputCommitter: could not write summary file for file:/tmp/goods/2015-6 java.lang.NullPointerException at org.apache.parquet.hadoop.ParquetFileWriter.mergeFooters(ParquetFileWriter.java:456) at org.apache.parquet.hadoop.ParquetFileWriter.writeMetadataFile(ParquetFileWriter.java:420) at org.apache.parquet.hadoop.ParquetOutputCommitter.writeMetaDataFile(ParquetOutputCommitter.java:58) at org.apache.parquet.hadoop.ParquetOutputCommitter.commitJob(ParquetOutputCommitter.java:48) at org.apache.spark.sql.execution.datasources.BaseWriterContainer.commitJob(WriterContainer.scala:230) ```
