function callbacks cannot be stored

As the doc(https://developer.garmin.com/connect-iq/monkey-c/functions/#Callbacks) says, function can be transmited as callback, The transmitation works, but the callback cannot be stored in a "var" variable.

Demo as follows:

/*************AppView***************/
class AppView extends WatchUi.View {

  protected var myClassInst;

  function initialize() {
     View.initialize();
     myClassInst= new MyClass(method(:requestUpdate)); // works, an object having "mMethod:requestUpdate" is transmitted
  }
}




/*************MyClass***************/
class MyClass{
  private var _theCallback;
  protected var tickTimer;

  function initialize(callbackFun) {
    _theCallback=callbackFun; // according to the debug info, "callbackFun" value is the same as transmitted, but  "_theCallback" shows "not available"
    tickTimer.start(method(:tickTimerCallback), 1000, true); // start a timer to invoke callback functions
  }

  function tickTimerCallback() {
    (_theCallback&& _theCallback.invoke());  //   "_theCallback" shows "not available" too, and the update requested not occurs
  }
}

Appreciated for your answer !