in RPGILE I get a java.lang.NullPointerException

["marcd" <marcd@xxxxxxxxxxxxxxxxxxxxx>]

The error I get is a "java.lang.NullPointerException"
On the call to the constructor
 
The .JAR can be had @ http://jakarta.apache.org/poi/ 
My code is:
 
DWB                      S        O   CLASS(*JAVA                    
D
:'org.apache.poi.hssf.usermodel-
D                                           .HSSFWorkbook')

DCREATEWB         PR      O   EXTPROC(*JAVA                   
D
:'org.apache.poi.hssf.usermodel-
D                                           .HSSFWorkbook'

D                                           :*CONSTRUCTOR)

  /FREE

  // Create a Workbook.                                                
  WB = CREATEWB();                                                      
  *INLR = *ON;

  /END-FREE              
 
Additionally,  
 
Is there a way I can generate a java stack trace within rpg, so that I
can figure out exactly
what goes wrong next time?