fix(android): unsubscribe click listener when Flutter engine detaches (#1140)

Author: nan-li

android/src/main/java/com/onesignal/flutter/OneSignalNotifications.java

@@ -227,6 +227,12 @@ public void onNotificationPermissionChange(boolean permission) {
         invokeMethodOnUiThread("OneSignal#onNotificationPermissionDidChange", hash);
     }
 
+    void onDetachedFromEngine() {
+        // Unsubscribe so clicks while the engine is dead get queued by the native SDK
+        // instead of dispatched on a detached channel.
+        OneSignal.getNotifications().removeClickListener(this);
+    }
+
     private void lifecycleInit(Result result) {
         OneSignal.getNotifications().removeForegroundLifecycleListener(this);
         OneSignal.getNotifications().addForegroundLifecycleListener(this);

android/src/main/java/com/onesignal/flutter/OneSignalPlugin.java

@@ -48,7 +48,9 @@ public void onDetachedFromEngine(@NonNull FlutterPlugin.FlutterPluginBinding bin
         onDetachedFromEngine();
     }
 
-    private void onDetachedFromEngine() {}
+    private void onDetachedFromEngine() {
+        OneSignalNotifications.getSharedInstance().onDetachedFromEngine();
+    }
 
     @Override
     public void onAttachedToActivity(@NonNull ActivityPluginBinding binding) {