Why is my listbox empty?

Dear all,

It looks that simple, the array is filled, the list box has its items - but the display is empty:

Status of script at breakpoint:

KLD_Z.main = function () { 
var asColours = [], oDoc, wPalC;
  oDoc = app.ActiveDoc;
  wPalC = new Window("palette", "Colours in this document");  
  wPalC.lis1 = wPalC.add ("listbox", undefined, undefined);
  wPalC.lis1.preferredSize.width = 210; 
  KLD_Z.GetColourNameList(oDoc, asColours);    // OK
  KLD_Z.FillDlgList (wPalC.lis1, asColours);   // OK
  wPalC.show();                                // NOK
$.bp(true);
} //--- end main -----------------------------------------

I have not the slightest idea what's wrong here.