Opening a window in XUL

Question

An exception is being thrown when trying to open a window using window.open in XUL. The exception details: Exception... "Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsIDOMJSWindow.open]" nsresult: "0x80004005 (NS_ERROR_FAILURE)" location:

The line of code throwing the exception is:

newWind = window.open(settings.url, "Dialog", "chrome = yes, width=" + 
          settings.width + ",height=" + settings.height + 
          ",resizable = yes,scrollbars=yes");

Can anyone please tell me how to get round this problem.