setCloseListener method Null safety

dynamic setCloseListener()

Implementation

setCloseListener() {
  if (widget.closeTag == null) return;
  if (globalMap[widget.closeTag] == null) {
    globalMap[widget.closeTag] = {};
  }
  var _controller = LeftScrollStatusCtrl();
  globalMap[widget.closeTag]![widget.key] = _controller;
  globalMap[widget.closeTag]![widget.key]!.addListener(handleChange);
}