Adding a breakpoint results in Exception

When I try to add a breakpoint, I get a NullPointerException at

org.netbeans.modules.debugger.support.DebuggerSupport.addBreakpoint(DebuggerSup port.java:450)

Using the menu or the button, I get the dialog "Add Breakpoint", before. Clicking on the margin throws the exception directly.

I tried j2sdk1.4.0_03 and j2sdk1.4.1_02 ; no difference.

Any thoughts/hints? Am I missing some very basic configuration, here?

Thanks

Ulf

[473 byte] By [704616] at [2007-11-25 9:44:21]