Null Pointer

[grabelle]

Hi I get an error if I don't provide the visible colums.
(see stack trace). If I provide them, I just get an empty file.
Could you please help.$
thx

java.lang.NullPointerException
at org.vaadin.haijian.filegenerator.FileBuilder.buildColumnHeaders(FileBuilder.java:80)
at org.vaadin.haijian.filegenerator.FileBuilder.buildFileContent(FileBuilder.java:70)
at org.vaadin.haijian.filegenerator.FileBuilder.getFile(FileBuilder.java:53)
at org.vaadin.haijian.Exporter.getStream(Exporter.java:101)
at com.vaadin.server.StreamResource.getStream(StreamResource.java:141)
at com.vaadin.server.FileDownloader.handleConnectorRequest(FileDownloader.java:140)
at com.vaadin.server.ConnectorResourceHandler.handleRequest(ConnectorResourceHandler.java:83)