"Error calling method on NPObject!"

[peterbe]

I sometimes get this error when using Flashsocket in Firefox. It's quite strange (stinks of Flash "brittleness") because I think a restart of the browser will fix the problem.

Error calling method on NPObject!
[Break On This Error] WebSocket.__flash.setCallerUrl(location.href); 

The line where is happens is this:

  /**
   * Called by Flash to notify JS that it's fully loaded and ready
   * for communication.
   */
  WebSocket.__onFlashInitialized = function() {
    // We need to set a timeout here to avoid round-trip calls
    // to flash during the initialization process.
    setTimeout(function() {
      WebSocket.__flash = document.getElementById("webSocketFlash");
      WebSocket.__flash.setCallerUrl(location.href);
      WebSocket.__flash.setDebug(!!window.WEB_SOCKET_DEBUG);
      for (var i = 0; i < WebSocket.__tasks.length; ++i) {
        WebSocket.__tasks[i]();
      }
      WebSocket.__tasks = [];
    }, 0);
  };

I guess it means that document.getElementById("webSocketFlash") returns null. But why?