The following assertion was thrown while routing a pointer event:
'package:flutter/src/services/platform_views.dart': Failed assertion: line 320 pos 15: 'downTime != null': is not true.


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      new AndroidMotionEvent (package:flutter/src/services/platform_views.dart:320:15)
#3      _MotionEventsDispatcher.dispatchPointerEvent (package:flutter/src/rendering/platform_view.dart:633:51)
#4      _MotionEventsDispatcher.handlePointerEvent (package:flutter/src/rendering/platform_view.dart:577:5)
#5      _PlatformViewGestureRecognizer.handleEvent (package:flutter/src/rendering/platform_view.dart:511:7)
#6      PointerRouter._dispatch (package:flutter/src/gestures/pointer_router.dart:76:12)
...
Event: PointerCancelEvent#df5cb(position: Offset(0.0, 0.0), localPosition: Offset(0.0, -348.3), pointer: 41, kind: touch, down: false, pressure: 0.0)

import 'package:flutter/foundation.dart';
import 'package:flutter/gestures.dart';
import 'package:flutter/material.dart';
import 'package:webview_flutter/webview_flutter.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('First Route'),
        ),
        body: ListView(
          children: <Widget>[
            SizedBox(
              height: 500,
              child: WebView(
                initialUrl: 'https://flutter.io',
                javascriptMode: JavascriptMode.unrestricted,
                gestureRecognizers: {
                  Factory<PlatformViewVerticalGestureRecognizer>(
                    () => PlatformViewVerticalGestureRecognizer()
                      ..onUpdate = (_) {},
                  ),
                },
              ),
            ),
            DropdownButton(
              items: [
                DropdownMenuItem(
                  child: Text('first'),
                ),
                DropdownMenuItem(
                  child: Text('second'),
                )
              ],
              onChanged: (value) {},
            )
          ],
        ),
      ),
    );
  }
}

class PlatformViewVerticalGestureRecognizer
    extends VerticalDragGestureRecognizer {
  PlatformViewVerticalGestureRecognizer({PointerDeviceKind kind})
      : super(kind: kind);

  Offset _dragDistance = Offset.zero;

  @override
  void addPointer(PointerEvent event) {
    startTrackingPointer(event.pointer);
  }

  @override
  void handleEvent(PointerEvent event) {
    _dragDistance = _dragDistance + event.delta;
    if (event is PointerMoveEvent) {
      final double dy = _dragDistance.dy.abs();
      final double dx = _dragDistance.dx.abs();

      if (dy > dx && dy > kTouchSlop) {
        // vertical drag - accept
        resolve(GestureDisposition.accepted);
        _dragDistance = Offset.zero;
      } else if (dx > kTouchSlop && dx > dy) {
        resolve(GestureDisposition.accepted);
        // horizontal drag - stop tracking
        stopTrackingPointer(event.pointer);
        _dragDistance = Offset.zero;
      }
    }
  }

  @override
  String get debugDescription => 'horizontal drag (platform view)';

  @override
  void didStopTrackingLastPointer(int pointer) {}
}```

Reproduction steps:
1- Use 2 fingers to pinch on the webview portion, as if you're zooming in/out (zoom doesn't work on Android so it will not react).
2- tap on the dropdown button while you are still pinching with 2 fingers on the webview.

Tested on OnePlus 5T, Android 8.1