title: Null 'key' argument error
body: <p>I'm using IReport 3.5.0 and my java GWT application uses the compiled .jasper file, to create a report. I'm creating the datasource in the java application and use <code>fillReport()</code> method to fill the report. It works fine, it fills the fields that I have defined both in the .jrxml and Java application.</p>
  
  <p>However, I want to use those fields to create a pie chart, and it doesn't work. I give my working fields as key and value expressions for the chart, but when I run the application, it gives the error <code>java.lang.IllegalArgumentException: Null 'key' argument.</code> I couldn't solve it, even when I give <code>$V{PAGE_COUNT}</code> as key expression, it always give the same 'null key' error. I'm stuck. Thanks for any help.</p>