on adding layers "Oops, something went wrong... Sorry, something unexpected happened on the server."

Resolving as cannot reproduce due to lack of necessary information to work on it.

Looks like there is a dangling reference in the store list, a ModificationProxy pointing to null. But yes, we need more context to understand this issue and figure out how one got there.
Did it happen again or maybe after a restart it stopped occurring?

@Biyas does this affect all layers and stores? What is your GeoServer version and Java version? What web browser did you use?

We really need more information if we are to investigate this report. While the exception stack trace is useful, it is insufficient for us to reproduce this failure.