APPOLOVIN ERROR com.applovin.mediation.hybridAds.MaxHybridNativeAdActivity.onCreate

Question

I meet some crashes in my android game. can anyone can fix this? here is the stack flow at top

Fatal Exception: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.drs.star/com.applovin.mediation.hybridAds.MaxHybridNativeAdActivity}: java.lang.IllegalArgumentException: Cannot add a null child view to a ViewGroup
       at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2951)
       at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3086)
       at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
       at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
       at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1816)
       at android.os.Handler.dispatchMessage(Handler.java:106)
       at android.os.Looper.loop(Looper.java:193)
       at android.app.ActivityThread.main(ActivityThread.java:6718)
       at java.lang.reflect.Method.invoke(Method.java)
       at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

it seem like the crash happen when the application want to play a advertise video. can any one help?

the following is my code,it is in my jar plugin, put into the Plugins/Android folder in unity

    public static void ShowAd() {
        if (Instance.rewardedAd != null) {
            if (Instance.rewardedAd.isReady()) {
                currentActivity.runOnUiThread(new Runnable() {
                    public void run() {
                        ApplovinHelper.Instance.rewardedAd.showAd();
                    }
                });
            } else {
                Log.d("ApplovinHelper", "Rewarded is NotReady");
                Toast.makeText(currentActivity, "Rewarded is NotReady,Try again later", 0).show();
            }
        } else {
            Log.d("ApplovinHelper", "Rewarded ad missing");
        }
    }

waht can i do?

i have no idea what to do