How to get present number of inserts/widgets in an overlay?

Question

I have created flat buttons that hide or show widget by inserting and removing widget in overlay. And if I keep pressing the hide button even after there are no more widgets left in overlay, I get

'_overlay != null': is not true.

Now, I wish to add a check in there but I didn't find any way to check present number of widgets in Overlay stack. Any help?

void showOverlayEntry() {
    createOverlayEntry();
    Overlay.of(context).insert(overlayEntry);
  }

void removeOverlayEntry() {
//wish to add a check here if there are no more inserts left to remove
    overlayEntry.remove();
  }