Android crash: java.lang.IllegalStateException: Reply already submitted

## What did you do?

Click button and call 
``` 
final LoginResult result = await LineSDK.instance.login(
        scopes: ["profile", "openid", "email"],
      );
 ```
`and press Back button of android device quickly.`

## What did you expect?

Cancel the login-request.

## What happened actually?

> Crash.

## Your environment?

flutter_Line_sdk-1.2.6
android os 8.1.0