Anomaly Not_found with empty custom entry

[Alizter]

Description of the problem

Empty custom entries throw a not found exception if used, we should probably catch this and print an error.

Declare Custom Entry FOO.

Axiom bar : Type -> Type.

Notation "[> s <]" := (bar s)
  (s custom FOO at level 105,
  format "[>  s  <]").

Goal forall s, [> s <].

Anomaly
"Uncaught exception Not_found."
Please report at http://coq.inria.fr/bugs/.
Raised at Stdlib__hashtbl.find in file "hashtbl.ml", line 539, characters 13-28
Called from LStream.get_loc in file "lib/lStream.ml" (inlined), line 54, characters 2-22
Called from Gramlib__Grammar.GMake.Parsable.parse_parsable.get_parsing_loc in file "gramlib/grammar.ml", line 1614, characters 16-53
Called from Gramlib__Grammar.GMake.Parsable.parse_parsable in file "gramlib/grammar.ml", line 1622, characters 16-34
Re-raised at Exninfo.iraise in file "clib/exninfo.ml", line 81, characters 4-38
Called from Flags.with_modified_ref in file "lib/flags.ml", line 17, characters 14-17
Re-raised at Exninfo.iraise in file "clib/exninfo.ml", line 81, characters 4-38
Called from Dune__exe__Idetop.add in file "ide/coqide/idetop.ml", line 82, characters 8-64
Called from Dune__exe__Idetop.eval_call.interruptible in file "ide/coqide/idetop.ml", line 443, characters 12-15
Called from Xmlprotocol.abstract_eval_call in file "ide/coqide/protocol/xmlprotocol.ml", line 690, characters 29-44

Coq Version

8.15
c70e317