Attempt to remove non-JNI local reference, dumping thread

## System info

Issue occurs on: tested only on Android
Plugin version: 0.3.8
Flutter doctor output:

```
[✓] Flutter (Channel stable, v1.9.1+hotfix.2, on Linux, locale pt_BR.UTF-8)

[✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
[!] Android Studio (not installed)
[✓] VS Code (version 1.38.1)
[✓] Connected device (1 available)
```

## Steps to Reproduce

1. when screen shows up

## Logs

```
W/art ( 2809): Attempt to remove non-JNI local reference, dumping thread
W/art ( 2809): Attempt to remove non-JNI local reference, dumping thread
W/art ( 2809): Attempt to remove non-JNI local reference, dumping thread
W/art ( 2809): Attempt to remove non-JNI local reference, dumping thread
W/art ( 2809): Attempt to remove non-JNI local reference, dumping thread
W/art ( 2809): Attempt to remove non-JNI local reference, dumping thread
W/art ( 2809): Attempt to remove non-JNI local reference, dumping thread
W/art ( 2809): Attempt to remove non-JNI local reference, dumping thread
W/art ( 2809): Attempt to remove non-JNI local reference, dumping thread
I/art ( 2809): Background sticky concurrent mark sweep GC freed 29(752B) AllocSpace objects, 0(0B) LOS objects, 0% free, 13MB/13MB, paused 5.379ms total 7.415ms
```
and keep printing it in an infinite loop...

```
✓] Flutter (Channel stable, v1.9.1+hotfix.2, on Linux, locale pt_BR.UTF-8)
• Flutter version 1.9.1+hotfix.2 at /home/rodrigo/Development/flutter
• Framework revision 2d2a1ffec9 (2 weeks ago), 2019-09-06 18:39:49 -0700
• Engine revision b863200c37
• Dart version 2.5.0

[✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
• Android SDK at /home/rodrigo/Android/SDK
• Android NDK location not configured (optional; useful for native profiling support)
• Platform android-28, build-tools 28.0.3
• ANDROID_HOME = /home/rodrigo/Android/SDK
• Java binary at: /usr/bin/java
• Java version OpenJDK Runtime Environment (build 1.8.0_222-8u222-b10-1~deb9u1-b10)
• All Android licenses accepted.

[!] Android Studio (not installed)
• Android Studio not found; download from https://developer.android.com/studio/index.html
(or visit https://flutter.dev/setup/#android-setup for detailed instructions).

[✓] VS Code (version 1.38.1)
• VS Code at /usr/share/code
• Flutter extension version 3.4.1

[✓] Connected device (1 available)
• Custom Phone • 192.16[...]:5555 • android-x86 • Android 6.0 (API 23)
```

Tái hiện cảnh báo trên Android với flutter_webview_plugin 0.3.8 bằng cách sử dụng thiết lập đã được báo cáo, bắt đầu từ khi màn hình xuất hiện. Kiểm tra phần xử lý Java/JNI native của plugin và sử dụng log ART lặp lại làm tín hiệu lỗi. Hoàn tất khi cảnh báo không còn được in liên tục lúc WebView mở.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.