Amplify Auth : Unable to add User to the Pool

I am trying to use Amplify Auth to add users to Cognito User Pool. I wrote this simple code, but even it doesn't work:

try {
    Amplify.addPlugin(new AWSCognitoAuthPlugin());
} catch (AmplifyException e) {
    e.printStackTrace();
}

Amplify.Auth.signUp(
    "dragon",
    "Password123@",
    AuthSignUpOptions.builder()
        .userAttribute(AuthUserAttributeKey.email(), "myemailaddress@gmail.com")
        .build(),
    result -> Log.i("AuthQuickStart", "Result: " + result.toString()),
    error -> Log.e("AuthQuickStart", "Sign up failed", error)
);

It prints this error to logcat:

E/AuthQuickStart: Sign up failed
    AmplifyException {message=Sign up failed, cause=java.lang.NullPointerException: Attempt to invoke virtual method 'void com.amazonaws.mobileconnectors.cognitoidentityprovider.CognitoUserPool.signUp(java.lang.String, java.lang.String, com.amazonaws.mobileconnectors.cognitoidentityprovider.CognitoUserAttributes, java.util.Map, com.amazonaws.mobileconnectors.cognitoidentityprovider.handlers.SignUpHandler)' on a null object reference, recoverySuggestion=See attached exception for more details}
        at com.amplifyframework.auth.cognito.AWSCognitoAuthPlugin$2.onError(AWSCognitoAuthPlugin.java:1)
        at com.amazonaws.mobile.client.internal.InternalCallback.call(InternalCallback.java:77)
        at com.amazonaws.mobile.client.internal.InternalCallback.access$000(InternalCallback.java:34)
        at com.amazonaws.mobile.client.internal.InternalCallback$1.run(InternalCallback.java:103)
        at java.lang.Thread.run(Thread.java:764)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.amazonaws.mobileconnectors.cognitoidentityprovider.CognitoUserPool.signUp(java.lang.String, java.lang.String, com.amazonaws.mobileconnectors.cognitoidentityprovider.CognitoUserAttributes, java.util.Map, com.amazonaws.mobileconnectors.cognitoidentityprovider.handlers.SignUpHandler)' on a null object reference
        at com.amazonaws.mobile.client.AWSMobileClient$13.run(AWSMobileClient.java:1894)
        at com.amazonaws.mobile.client.internal.InternalCallback$1.run(InternalCallback.java:101)
        at java.lang.Thread.run(Thread.java:764)