_ElementLifecycle.defunct': is not true.

[ryanhz]

Either the assertion indicates an error in the framework itself,
or we should provide substantially more information in this error
message to help you determine and fix the underlying cause.
In either case, please report this assertion by filing a bug on
GitHub:
https://github.com/flutter/flutter/issues/new?template=BUG.md

When the exception was thrown, this was the stack:
2 Element.markNeedsBuild (package:flutter/src/widgets/framework.dart:3641:12)
3 State.setState (package:flutter/src/widgets/framework.dart:1161:14)
4 _DropdownMenuState._handleShow (package:dropdown_menu/_src/dropdown_menu.dart:316:5)
5 _DropdownMenuState.onShow (package:dropdown_menu/_src/dropdown_menu.dart:310:12)
6 _DropdownMenuState.onEvent (package:dropdown_menu/_src/dropdown_menu.dart:355:11)
7 DropdownState._onController (package:dropdown_menu/_src/drapdown_common.dart:189:5)
8 ChangeNotifier.notifyListeners (package:flutter/src/foundation/change_notifier.dart:206:21)
9 DropdownMenuController.show (package:dropdown_menu/_src/drapdown_common.dart:39:5)
10 _DropdownHeaderState.buildItem. (package:dropdown_menu/_src/dropdown_header.dart:88:24)
11 GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:182:24)
12 TapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:365:11)
13 TapGestureRecognizer.handlePrimaryPointer (package:flutter/src/gestures/tap.dart:275:7)
14 PrimaryPointerGestureRecognizer.handleEvent (package:flutter/src/gestures/recognizer.dart:455:9)
15 PointerRouter._dispatch (package:flutter/src/gestures/pointer_router.dart:75:13)
16 PointerRouter.route (package:flutter/src/gestures/pointer_router.dart:102:11)
17 _WidgetsFlutterBinding&BindingBase&GestureBinding.handleEvent (package:flutter/src/gestures/binding.dart:218:19)
18 _WidgetsFlutterBinding&BindingBase&GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:198:22)
19 _WidgetsFlutterBinding&BindingBase&GestureBinding._handlePointerEvent (package:flutter/src/gestures/binding.dart:156:7)
20 _WidgetsFlutterBinding&BindingBase&GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:102:7)
21 _WidgetsFlutterBinding&BindingBase&GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:86:7)
25 _invoke1 (dart:ui/hooks.dart:250:10)
26 _dispatchPointerDataPacket (dart:ui/hooks.dart:159:5)
(elided 5 frames from class _AssertionError and package dart:async)

The DropdownMenuController sending notification was:
Instance of 'DropdownMenuController'