In-Context Checkout: Uncaught SecurityError: Blocked a frame with origin: checkout.js:4734 throws error - reactjs

Question

I'm implementing PayPal's In-Context Checkout and am using Advanced In-Context JavaScript settings (https://developer.paypal.com/docs/classic/express-checkout/in-context/javascript_advanced_settings)
My app is a React app. So I can't use PP API as they suggest it, that is just throw a code between <script> ... </script> tags somewhere in the page under their buttons. My React components have state and data that I need to send to server within PP function calls. So I placed PP code in componentDidMount method. And for some reason PP throws this error:
checkout.js:4734 Uncaught SecurityError: Blocked a frame with origin "http://example.com:3000" from accessing a frame with origin "https://www.sandbox.paypal.com". The frame requesting access has a protocol of "http", the frame being accessed has a protocol of "https". Protocols must match.
(anonymous function) # checkout.js:4734
checkout.js:4515 GET http://www.paypalobjects.com/api/oneTouch.html net::ERR_EMPTY_RESPONSE
Here is the code:
componentDidMount() {
window.paypalCheckoutReady = function() {
paypal.checkout.setup(config[NODE_ENV].ppMerchantID, {
locale: 'en_US',
environment: 'sandbox',
buttons: [
{
container: 'checkoutBtnContainer',
type: 'checkout',
color: 'gold',
size: 'medium',
shape: 'pill',
click: (ev)=>{
paypal.checkout.initXO();
$.post('/checkout', {
checkoutData: this.props.checkoutData,
})
.done(res => {
paypal.checkout.startFlow(res.link);
})
.fail(err => {
paypal.checkout.closeFlow();
});
}
}
],
});
};
},
I know about cross-origin policy. I don't understand why it is the case here. Why the code works fine if I throw it on the page between <script> ... </script> tags, but PP throws an error if I use it in my React component. What is the cause of that? Is it React fault or PayPal's?

Answer 1

UPD: No, below is not a solution for the problem. Occasionally Paypal's checkout.js script throws the error.
However, it solves this issue
Apparently,
1) there was no this:
window.paypalCheckoutReady = function() {
// wrong this is here
}
I changed to:
window.paypalCheckoutReady = () => {
// correct this is here now
}
I don't like .bind(this).
2) I removed <form /> tag and set plain <div> instead:
// before
<form id="checkoutBtnContainer" method="post" action="/checkout"></form>
// after
<div id="checkoutBtnContainer"></div>
I don't know why and how, but all works fine now.