showLoadingEntry method
显示Loading Entry
Implementation
void showLoadingEntry(OverlayEntry entry) {
  final overlay = _getOverlay();
  if (overlay == null) {
    Log.console('ComLoading: 无法找到可用的Overlay');
    return;
  }
  // 如果已经有Loading在显示,直接返回
  if (_isLoadingShowing) return;
  _isLoadingShowing = true;
  _loadingEntry = entry;
  overlay.insert(entry);
}