In many cases when reading undo memfile, n the 'restore id from old
main' part of the process, the 'id_old' is not set, which means that
the call to `BKE_main_idmap_insert_id` would try to dereference a `nullptr`.
In practice this is likely not an issue (see comment in code for details),
but at least explicitely check for a nullptr `id_old` pointer.
Would deffer actual cleanup of this area for after 3.5 is branched out.
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