아래 코드에서 해쉬 결과값을 얻은 후 내 어플리케이션 -> 앱 설정 -> 플랫폼 -> Android 해쉬 키 수정 또는 추가하시면 됩니다.

import android.content.pm.PackageInfo;
import android.content.pm.PackageManager;
import android.content.pm.Signature;
import android.os.Bundle;
import android.util.Base64;
import android.util.Log;

import androidx.annotation.Nullable;

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

import io.flutter.embedding.android.FlutterActivity;

public class MainActivity extends FlutterActivity {

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        getHashKey();
    }

    private void getHashKey()
    {
        PackageInfo packageInfo = null;
        try
        {
            packageInfo = getPackageManager().getPackageInfo(getPackageName(), PackageManager.GET_SIGNATURES);
        }
        catch (PackageManager.NameNotFoundException e)
        {
            e.printStackTrace();
        }
        if (packageInfo == null)
            Log.e("KeyHash", "KeyHash:null");

        for (Signature signature : packageInfo.signatures)
        {
            try
            {
                MessageDigest md = MessageDigest.getInstance("SHA");
                md.update(signature.toByteArray());
                Log.e("KeyHash", Base64.encodeToString(md.digest(), Base64.DEFAULT));
            }
            catch (NoSuchAlgorithmException e)
            {
                Log.e("KeyHash", "Unable to get MessageDigest. signature=" + signature, e);
            }
        }
    }
}