we are using a Custom JasperReports Datasource Provider as Data Adapter.
If the report contains an error, i.e. in the body there’s the reference to a not existent field, the Read Fields button in the dataset and query dialog doesn’t work, and we get the following exception
java.lang.ClassCastException: net.sf.jasperreports.engine.design.JRValidationFault cannot be cast to net.sf.jasperreports.engine.design.JRValidationFault
In attachment you have a doc with more info and a sample workspace where the problem has been reproduced.
Please let me know if you need more info.