Readyevent must be triggered twice

Hey,
I am using the readyEvent to trigger backstopJS to take the screenshots but I always get TimeoutErrors when i'm not triggering it twice.
```
window.addEventListener('load', () => {
// Backstop must be triggered twice.
console.log('pageLoaded');
setTimeout(() => {
console.log('pageLoaded');
}, 1000);
});
```
I am using the docker image v3.0.38