Plugin not working

[MicroProgramersSoftware]

Hi, I'm trying to use this plugin in my flutter app. Also followed all the steps mentioned to import the plugin. And it's successfully imported too.
But I'm getting this exception everytime:

E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): Failed to handle method call
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): java.lang.ClassCastException: lab.neruno.flutter_ipay88_my.IPayChannelDelegate cannot be cast to java.io.Serializable
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at com.ipay.IPayIH.checkout(IPayIH.java:43)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at lab.neruno.flutter_ipay88_my.FlutterIpay88MyPlugin.onMethodCall(FlutterIpay88MyPlugin.kt:54)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at io.flutter.plugin.common.MethodChannel$IncomingMethodCallHandler.onMessage(MethodChannel.java:262)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at io.flutter.embedding.engine.dart.DartMessenger.invokeHandler(DartMessenger.java:295)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at io.flutter.embedding.engine.dart.DartMessenger.lambda$dispatchMessageToQueue$0$io-flutter-embedding-engine-dart-DartMessenger(DartMessenger.java:319)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at io.flutter.embedding.engine.dart.DartMessenger$$ExternalSyntheticLambda0.run(Unknown Source:12)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at android.os.Handler.handleCallback(Handler.java:938)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at android.os.Handler.dispatchMessage(Handler.java:99)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at android.os.Looper.loopOnce(Looper.java:233)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at android.os.Looper.loop(Looper.java:334)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at android.app.ActivityThread.main(ActivityThread.java:8333)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at java.lang.reflect.Method.invoke(Native Method)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:582)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1065)
E/DartMessenger( 8055): Uncaught exception in binary message listener
E/DartMessenger( 8055): java.lang.IllegalStateException: Reply already submitted
E/DartMessenger( 8055): 	at io.flutter.embedding.engine.dart.DartMessenger$Reply.reply(DartMessenger.java:430)
E/DartMessenger( 8055): 	at io.flutter.plugin.common.MethodChannel$IncomingMethodCallHandler.onMessage(MethodChannel.java:282)
E/DartMessenger( 8055): 	at io.flutter.embedding.engine.dart.DartMessenger.invokeHandler(DartMessenger.java:295)
E/DartMessenger( 8055): 	at io.flutter.embedding.engine.dart.DartMessenger.lambda$dispatchMessageToQueue$0$io-flutter-embedding-engine-dart-DartMessenger(DartMessenger.java:319)
E/DartMessenger( 8055): 	at io.flutter.embedding.engine.dart.DartMessenger$$ExternalSyntheticLambda0.run(Unknown Source:12)
E/DartMessenger( 8055): 	at android.os.Handler.handleCallback(Handler.java:938)
E/DartMessenger( 8055): 	at android.os.Handler.dispatchMessage(Handler.java:99)
E/DartMessenger( 8055): 	at android.os.Looper.loopOnce(Looper.java:233)
E/DartMessenger( 8055): 	at android.os.Looper.loop(Looper.java:334)
E/DartMessenger( 8055): 	at android.app.ActivityThread.main(ActivityThread.java:8333)
E/DartMessenger( 8055): 	at java.lang.reflect.Method.invoke(Native Method)
E/DartMessenger( 8055): 	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:582)
E/DartMessenger( 8055): 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1065)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): Failed to handle method call
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): java.lang.ClassCastException: lab.neruno.flutter_ipay88_my.IPayChannelDelegate cannot be cast to java.io.Serializable
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at com.ipay.IPayIH.requery(IPayIH.java:51)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at lab.neruno.flutter_ipay88_my.FlutterIpay88MyPlugin.onMethodCall(FlutterIpay88MyPlugin.kt:68)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at io.flutter.plugin.common.MethodChannel$IncomingMethodCallHandler.onMessage(MethodChannel.java:262)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at io.flutter.embedding.engine.dart.DartMessenger.invokeHandler(DartMessenger.java:295)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at io.flutter.embedding.engine.dart.DartMessenger.lambda$dispatchMessageToQueue$0$io-flutter-embedding-engine-dart-DartMessenger(DartMessenger.java:319)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at io.flutter.embedding.engine.dart.DartMessenger$$ExternalSyntheticLambda0.run(Unknown Source:12)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at android.os.Handler.handleCallback(Handler.java:938)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at android.os.Handler.dispatchMessage(Handler.java:99)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at android.os.Looper.loopOnce(Looper.java:233)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at android.os.Looper.loop(Looper.java:334)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at android.app.ActivityThread.main(ActivityThread.java:8333)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at java.lang.reflect.Method.invoke(Native Method)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:582)
E/MethodChannel#my.lab.neruno.ipay88/platform( 8055): 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1065)
E/DartMessenger( 8055): Uncaught exception in binary message listener
E/DartMessenger( 8055): java.lang.IllegalStateException: Reply already submitted
E/DartMessenger( 8055): 	at io.flutter.embedding.engine.dart.DartMessenger$Reply.reply(DartMessenger.java:430)
E/DartMessenger( 8055): 	at io.flutter.plugin.common.MethodChannel$IncomingMethodCallHandler.onMessage(MethodChannel.java:282)
E/DartMessenger( 8055): 	at io.flutter.embedding.engine.dart.DartMessenger.invokeHandler(DartMessenger.java:295)
E/DartMessenger( 8055): 	at io.flutter.embedding.engine.dart.DartMessenger.lambda$dispatchMessageToQueue$0$io-flutter-embedding-engine-dart-DartMessenger(DartMessenger.java:319)
E/DartMessenger( 8055): 	at io.flutter.embedding.engine.dart.DartMessenger$$ExternalSyntheticLambda0.run(Unknown Source:12)
E/DartMessenger( 8055): 	at android.os.Handler.handleCallback(Handler.java:938)
E/DartMessenger( 8055): 	at android.os.Handler.dispatchMessage(Handler.java:99)
E/DartMessenger( 8055): 	at android.os.Looper.loopOnce(Looper.java:233)
E/DartMessenger( 8055): 	at android.os.Looper.loop(Looper.java:334)
E/DartMessenger( 8055): 	at android.app.ActivityThread.main(ActivityThread.java:8333)
E/DartMessenger( 8055): 	at java.lang.reflect.Method.invoke(Native Method)
E/DartMessenger( 8055): 	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:582)
E/DartMessenger( 8055): 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1065)
V/ImeFocusController( 8055): onWindowFocus: DecorView@461220f[MainActivity] softInputMode=STATE_UNSPECIFIED|ADJUST_RESIZE|IS_FORWARD_NAVIGATION
V/ImeFocusController( 8055): Restarting due to isRestartOnNextWindowFocus as true
D/ImeFocusController( 8055): onViewFocusChanged, view=DecorView@461220f[MainActivity], mServedView=null
V/ImeFocusController( 8055): checkFocus: view=null next=DecorView@461220f[MainActivity] force=true package=<none>

Please check this and guide me if I'm doing something wrong.