Sending `AppExit` during a startup schedule causes crash

## Bevy version

0.15.0

## Relevant system information

`AdapterInfo { name: "NVIDIA GeForce GTX 1060 6GB", vendor: 4318, device: 7171, device_type: DiscreteGpu, driver: "NVIDIA", driver_info: "535.183.01", backend: Vulkan }`

## What you did

Send an `AppExit` event during a startup schedule.

```
use bevy::prelude::*;

fn main() {
App::new()
.add_plugins(DefaultPlugins)
.add_systems(PreStartup, |mut event_writer: EventWriter<_>| {
event_writer.send(AppExit::Success);
})
.run();
}
```

## What went wrong

Sometimes it's a segfault, sometimes it's a double free, sometimes everything seems to be fine.

## Additional information

Running in a debugger suggests that bevy render is doing something wrong with the wgpu crate.