As spotted in#32719, the Serializer component is unable to deserialize someFlattenException instances, which breaks the messenger component when used with the symfony serializer.

While this can't be fixed inFlattenException without breaking BC, as the main/sole purpose of this class is to be serialization friendly, I thought it was a real issue.

Also, it should only break classes that extendFlattenException and I couldn't find a single project on Github that does this, nor imagine why anyone would want to (actually, I believe this class should have been marked final).

Here are all the issues I identified, and how they are fixed by this PR:

• getHeaders can returnnull orarray, butsetHeaders had anarray type-hint: I removed the type hint
• getPrevious can returnnull orself, butsetPrevious had aself type-hint: I removed the type-hint
• setTrace required 3 arguments, and added a new frame to the trace: the file and line arguments are now optional, and I try to detect whether the passed trace has already been processed or not. This may not be the best thing to do IMHO, but an attempt to limit the BC break to subclasses and ensure usage of the class doesn't get broken.

Note: The same issues affect theFlattenException class of the new ErrorRenderer component, but I will submit an new issue to discuss the best solution here, as I would also like to discuss#32873 and propose an alternate solution to#32473.