"there is no reactor running" panic when system is quickly dropped about actix HOT 2 OPEN

Hi there! I'm encountering the same problem. As an additional detail I could add that the same crash happen when ctx.notify_later is used. It seems like both ctx.notify_later and ctx.run_internval crashed due to access to tokio::runtime::Handle::current().
Crash is also happen when directly accessing current tokio handle:

use actix::prelude::*;

struct MyActor;

impl Actor for MyActor {
    type Context = Context<Self>;
    
    fn started(&mut self, ctx: &mut Self::Context) {
        tokio::runtime::Handle::current(); // will be paniced
    }
}

fn main() {
    let system = actix::System::new();
    system.block_on(async {
        MyActor.start();
        // Next line fixes panic when uncommented: 
        // tokio::task::yield_now().await;
    });
}

from actix.

I believe this is caused by the Drop implementation of ContextFut:

The poll will call your started function if ContextFut didn't get the change to be polled before. The thing is, tokio drops its spawned futures after shutting down the runtime, run_interval will try to create a Sleep instance from tokio and then panic.

The Drop implementation is arguably wrong, the poll can run arbitrary code provided by the user (through started, stopping and stopped), and it's understandable that the user will assume that a tokio runtime exists in at least some of these methods.

from actix.