Getting an issue with 'executor must be set' about a2 HOT 4 CLOSED

use a2::{Client, Endpoint, NotificationBuilder, NotificationOptions, PlainNotificationBuilder};
use argparse::{ArgumentParser, Store, StoreOption, StoreTrue};
use pretty_env_logger;
use std::fs::File;
use tokio;

// An example client connectiong to APNs with a certificate and key
#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
    pretty_env_logger::init();

    let mut certificate_file = String::from("/Users/rajesh/Downloads/BlueshiftPushBluePkcs.p12");
    let mut password = String::from("blueshift");
    let mut device_token = String::from("860177d09c9a45900a20b1bf5de4eeddd4046b151bb837523d3218fdb135ead2");
    let mut message = String::from("Ch-check it out!");
    let mut sandbox = true;
    let mut topic: Option<String> = Some(String::from("com.blueshift.reads"));


    // Read the private key and certificate from the disk
    let mut certificate = File::open(certificate_file).unwrap();

    // Which service to call, test or production?
    let endpoint = if sandbox {
        Endpoint::Sandbox
    } else {
        Endpoint::Production
    };

    // Connecting to APNs using a client certificate
    let client = Client::certificate(&mut certificate, &password, endpoint).unwrap();

    let options = NotificationOptions {
        apns_topic: topic.as_ref().map(|s| &**s),
        ..Default::default()
    };

    // Notification payload
    let mut builder = PlainNotificationBuilder::new(message.as_ref());
    builder.set_sound("default");
    builder.set_badge(1u32);

    let payload = builder.build(device_token.as_ref(), options);
    println!("Sending payload");

    
    let response = client.send(payload).await?;

    println!("Sent: {:?}", response);

    Ok(())
}

    39: tokio::runtime::Runtime::block_on
         at /Users/franklin/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.5.0/src/runtime/mod.rs:452:43
    40: notifier::main
         at /Users/franklin/Developer/rust/Notifier/src/main.rs:9:1
    41: core::ops::function::FnOnce::call_once
         at /Users/franklin/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5