Summary

The PR fixes #29902
Wrapping part of createElement() function with additional check if config argument value is an object. This check makes sense, because the wrapped code assumes that config is an object, referring to its properties and using in operator for that.
With the following kind of code:

<button
    {...(isCountButtonActive && {
      onClick: () => setCount(currentCount => currentCount + 1)
    })}
>
    Increment count
</button>

config is false if isCountButtonActive === false, so that produces JS error mentioned in the issue. It's fixed by the above-mentioned check.

How did you test this change?

I've built react packages with my changes, replaced them in node_modules folder of https://github.com/undeletable/jsx-spread-with-react-19-rc app (app using legacy JSX transform; the code above is from there) and ran that app in dev mode.

Before the changes:

Same app after the changes: