[Process] kill() returned unexpected error 1 - iOS

## System info

Issue occurs on: iOS
Plugin version: 0.3.8 & 0.3.10+1
Device used : iPad Air 2
Device iOS Version : 13.3.1

Flutter doctor output:

```
Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel stable, v1.12.13+hotfix.8, on Mac OS X 10.14.5 18F132, locale fr-BE)
[✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
[✓] Xcode - develop for iOS and macOS (Xcode 11.2.1)
[✓] Android Studio (version 3.2)
[✓] IntelliJ IDEA Ultimate Edition (version 2019.1.2)
[✓] Connected device (1 available)

• No issues found!

```

## Steps to Reproduce

1. We edit the user agent to adapt the behavior of the website and we launch the webview
2. The line `[Process] kill() returned unexpected error 1` appears all the time and seems to reload the webview all the time, which kill the user agent and break the behavior and interceptions for the app.

## Logs

Here are the logs for a 2 minutes utilisation (some lines have been cut to avoid to be to long to read since it's the same line ...

```
2020-02-13 11:23:21.267815+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:23:21.280795+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:23:21.280914+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:23:21.283047+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:23:21.409267+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:23:21.446632+0100 MyApp [414:17909] [] tcp_input [C2.1:3] flags=[R] seq=1006332445, ack=0, win=0 state=CLOSED rcv_nxt=1006332445, snd_una=1523391102
2020-02-13 11:23:21.446854+0100 MyApp [414:17909] [] tcp_input [C2.1:3] flags=[R] seq=1006332445, ack=0, win=0 state=CLOSED rcv_nxt=1006332445, snd_una=1523391102
2020-02-13 11:23:21.450507+0100 MyApp [414:17908] 6.16.0 - [Firebase/Analytics][I-ACS023141]
2020-02-13 11:23:21.477095+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:23:21.477223+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:23:21.492547+0100 MyApp [414:17906] [] tcp_input [C3.1:3] flags=[R] seq=3826942167, ack=0, win=0 state=CLOSED rcv_nxt=3826942167, snd_una=3459830947
2020-02-13 11:23:21.492746+0100 MyApp [414:17906] [] tcp_input [C3.1:3] flags=[R] seq=3826942167, ack=0, win=0 state=CLOSED rcv_nxt=3826942167, snd_una=3459830947
2020-02-13 11:23:21.501743+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:23:21.513696+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:23:21.518754+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:23:21.580803+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:23:21.581048+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:23:21.582360+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:23:21.617912+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:23:21.656890+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:23:21.657226+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:23:21.680600+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:23:21.711224+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:23:21.718833+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:23:21.785660+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:23:21.785954+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:23:21.810774+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:23:21.830742+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
… (Cut to reduce the length of lines) …
2020-02-13 11:25:55.184043+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:55.193870+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:55.685170+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:55.693938+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:56.186840+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:56.193856+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:56.688260+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:56.694009+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:57.193048+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:57.210668+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:57.666482+0100 MyApp [414:17686] VERBOSE: oneSignalApplicationWillResignActive
2020-02-13 11:25:57.667286+0100 MyApp [414:17686] VERBOSE: Application Backgrounded started
2020-02-13 11:25:57.671649+0100 MyApp [414:17686] VERBOSE: TimeProcessor for session attributed 0
2020-02-13 11:25:57.672312+0100 MyApp [414:17686] VERBOSE: sendOnFocusCall unattributed with totalTimeActive 216.483555
2020-02-13 11:25:57.672668+0100 MyApp [414:17686] VERBOSE: OSBaseFocusTimeProcessor hasMinSyncTime getMinSessionTime: 60 activeTime: 216.483555
2020-02-13 11:25:57.690065+0100 MyApp [414:18111] VERBOSE: beginBackgroundFocusTask start
2020-02-13 11:25:57.730550+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:57.731345+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:57.732619+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:57.743742+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:57.866528+0100 MyApp [414:17686] VERBOSE: sendOnFocusCallWithParams unattributed succeed, saveUnsentActiveTime with 0
2020-02-13 11:25:57.866660+0100 MyApp [414:17686] VERBOSE: OSUnattributedFocusTimeProcessor:endDelayBackgroundTask:12
2020-02-13 11:25:57.866939+0100 MyApp [414:17686] VERBOSE: endBackgroundFocusTask called
2020-02-13 11:25:58.192434+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:58.194243+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:58.693412+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:58.710581+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:59.196071+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:59.210716+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:59.328310+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:59.332942+0100 MyApp [414:17686] VERBOSE: oneSignalApplicationDidEnterBackground
2020-02-13 11:25:59.516527+0100 MyApp [414:17686] Can't end BackgroundTask: no background task exists with identifier 15 (0xf), or it may have already been ended. Break in UIApplicationEndBackgroundTaskError() to debug.
2020-02-13 11:25:59.516918+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:59.517251+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:59.518402+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:59.518723+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:25:59.522753+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:26:02.690795+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
2020-02-13 11:26:02.691814+0100 MyApp [414:17686] [Process] kill() returned unexpected error 1
```

#Flutter doctor -v output :
```
[✓] Flutter (Channel stable, v1.12.13+hotfix.8, on Mac OS X 10.14.5 18F132, locale fr-BE)
• Flutter version 1.12.13+hotfix.8 at /Users/dev/Desktop/Projects/flutter/flutter
• Framework revision 0b8abb4724 (2 days ago), 2020-02-11 11:44:36 -0800
• Engine revision e1e6ced81d
• Dart version 2.7.0

[✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
• Android SDK at /Users/dev/Library/Android/sdk
• Android NDK location not configured (optional; useful for native profiling support)
• Platform android-28, build-tools 28.0.3
• Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
• Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1136-b06)
• All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 11.2.1)
• Xcode at /Applications/Xcode.app/Contents/Developer
• Xcode 11.2.1, Build version 11B500
• CocoaPods version 1.7.5

[✓] Android Studio (version 3.2)
• Android Studio at /Applications/Android Studio.app/Contents
• Flutter plugin version 31.3.1
• Dart plugin version 181.5656
• Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1136-b06)

[✓] IntelliJ IDEA Ultimate Edition (version 2019.1.2)
• IntelliJ at /Applications/IntelliJ IDEA.app
• Flutter plugin version 35.3.3
• Dart plugin version 191.7141.49

[✓] Connected device (1 available)
• macdev’s iPad • 9d09a24ab86a38d0bd60a8fadd345d34510855c6 • ios • iOS 13.3.1

• No issues found!
```

Thanks by advance !