Firebase Crashlytics: java.util.ConcurrentModificationException

# Bug Report
Hi all,
I have an issue on Firebase Crash log,
Please help me check it, I am getting this crash frequently.
Thanks!

## Problem
*LinkedHashMap.java line 757, This issue has 44 crashes affecting 29 users (last 7 days)
```Fatal Exception: java.util.ConcurrentModificationException
Fatal Exception: java.util.ConcurrentModificationException
at java.util.LinkedHashMap$LinkedHashIterator.nextNode(LinkedHashMap.java:757)
at java.util.LinkedHashMap$LinkedValueIterator.next(LinkedHashMap.java:785)
at org.apache.cordova.PluginManager.postMessage(PluginManager.java:310)
at org.apache.cordova.CordovaWebViewImpl.postMessage(CordovaWebViewImpl.java:381)
at org.apache.cordova.splashscreen.SplashScreen$2.run(SplashScreen.java:166)
at android.os.Handler.handleCallback(Handler.java:873)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:6986)
at java.lang.reflect.Method.invoke(Method.java)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1445)
```

*LinkedHashMap.java line 760, This issue has 44 crashes affecting 29 users (last 7 days)
```
Fatal Exception: java.util.ConcurrentModificationException
at java.util.LinkedHashMap$LinkedHashIterator.nextNode(LinkedHashMap.java:760)
at java.util.LinkedHashMap$LinkedValueIterator.next(LinkedHashMap.java:788)
at org.apache.cordova.PluginManager.postMessage(PluginManager.java:310)
at org.apache.cordova.CordovaWebViewImpl.postMessage(CordovaWebViewImpl.java:381)
at org.apache.cordova.splashscreen.SplashScreen$2.run(SplashScreen.java:166)
at android.os.Handler.handleCallback(Handler.java:907)
at android.os.Handler.dispatchMessage(Handler.java:105)
at android.os.Looper.loop(Looper.java:216)
at android.app.ActivityThread.main(ActivityThread.java:7625)
at java.lang.reflect.Method.invoke(Method.java)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:524)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:987)
```

*LinkedHashMap.java line 775
```
java.util.LinkedHashMap$LinkedHashIterator.nextNode (LinkedHashMap.java:775)
java.util.LinkedHashMap$LinkedValueIterator.next (LinkedHashMap.java:803)
org.apache.cordova.PluginManager.postMessage (PluginManager.java:310)
org.apache.cordova.CordovaWebViewImpl.postMessage (CordovaWebViewImpl.java:381)
org.apache.cordova.splashscreen.SplashScreen$2.run (SplashScreen.java:166)
android.os.Handler.handleCallback (Handler.java:888)
android.os.Handler.dispatchMessage (Handler.java:100)
android.os.Looper.loop (Looper.java:213)
android.app.ActivityThread.main (ActivityThread.java:8147)
java.lang.reflect.Method.invoke (Method.java)
com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run (RuntimeInit.java:513)
com.android.internal.os.ZygoteInit.main (ZygoteInit.java:1101)
```

### Environment, Platform, Device
cordova-android 8.1.0
cordova-plugin-splashscreen 5.0.3 "Splashscreen"

Almost happens on Android 8+
Some devices: Galaxy S8, S9+, Galaxy A7(2017), Note 9, Xperia XZ Premium,...

### Codes

## Checklist

- [x] I searched for existing GitHub issues
- [x] I updated all Cordova tooling to most recent version
- [x] I included all the necessary information above

Start by tracing the crash at org.apache.cordova.PluginManager.postMessage line 310 through CordovaWebViewImpl.postMessage and the SplashScreen callback at SplashScreen.java line 166. Reproduce or investigate the ConcurrentModificationException on cordova-android 8.1.0 with cordova-plugin-splashscreen 5.0.3, then document the cause and verify that the reported Android crash path no longer occurs.

Written by the indexing model from the issue text.