showOverlay method
********* Overlay ********* /// 自定义Overlay
Implementation
/// 自定义Overlay
ExtendedOverlayEntry? showOverlay(Widget widget, {bool autoOff = false}) {
assert(FlExtended().navigatorKey.currentState != null,
'Set FlExtended().navigatorKey to one of [MaterialApp CupertinoApp WidgetsApp]');
final OverlayState? overlay =
FlExtended().navigatorKey.currentState!.overlay;
if (overlay == null) return null;
final ExtendedOverlayEntry entry =
ExtendedOverlayEntry(autoOff: autoOff, widget: widget);
overlay.insert(entry);
if (!autoOff) _overlayEntryList.add(entry);
return entry;
}