fix null pointer exception

src/main/java/utils/crawler/PFNData.java

@@ -65,7 +65,7 @@ public class PFNData {
 	 */
 	public String toCSV() {
 		final Collection<Object> values = data.values();
-		return values.stream().map(Object::toString).collect(Collectors.joining(","));
+		return values.stream().map(value -> value == null ? "null" : value.toString()).collect(Collectors.joining(","));
 	}
 
 	/**

src/main/java/utils/crawler/SEFileCrawler.java

@@ -565,7 +565,7 @@ public class SEFileCrawler {
 				pfns.add(pfnData.toJSON());
 			}
 			catch (final Exception e) {
-				logger.info("Cannot convert to CSV " + e.getMessage());
+				logger.info("Cannot convert to JSON " + e.getMessage());
 				e.printStackTrace();
 			}
 		}