### What
When rendering a parallel slot multiple times in a single layout, in
conjunction with using an error boundary, the following TypeError is
thrown:
> Cannot destructure property 'parallelRouterKey' of 'param' as it is
null
### Why
I'm not 100% sure of the reason, but I believe this is because of how
React attempts to dededupe (more specifically, "detriplficate") objects
that it sees getting passed across the RSC -> client component boundary
(and an error boundary is necessarily a client component). When React
sees the same object twice, it'll create a reference to that object and
then use that reference in future places where it sees the object. My
assumption is that there's a bug somewhere here, as the `LayoutRouter`
component for the subsequent duplicated parallel slots (after the first
one) have no props, hence the TypeError.
### How
Rather than passing the error component as a prop to `LayoutRouter`,
this puts it as part of the `CacheNodeSeedData` data structure. This is
more aligned with other properties anyway (such as `loading` and `rsc`
for each segment), and seems to work around this bug as the
`initialSeedData` prop is only passed from RSC->client once.
EDIT: Confirmed this is also fixed after syncing the latest React, due
to https://github.com/facebook/react/pull/28669Fixes#58485
Closes NEXT-2095
f93ae7c89e