TRIGGER may not see it

I created a trigger in my db with no errors, like this one:

CREATE OR REPLACE TRIGGER scadenziario_update_trigger

        BEFORE UPDATE ON scadenziario
        FOR EACH ROW
        DECLARE
            motivo_sopralluogo sopralluogo.cod_motivo_sopralluogo%%type;
        BEGIN
            SELECT sopralluogo.cod_motivo_sopralluogo INTO 
motivo_sopralluogo
            FROM scadenziario join sopralluogo on 
scadenziario.id_sopralluogo = sopralluogo.id
            WHERE scadenziario.id = :old.id ;
            IF motivo_sopralluogo = 'PRO' AND :new.id_piano IS NULL AND 
:old.id_piano IS NOT NULL  THEN
                RAISE_APPLICATION_ERROR(-20000, 'error trigger 212');
            END IF;
        END;

when I try this query:

UPDATE scadenziario
  SET id_veterinario_programmato=637
  WHERE id_unita_aziendale=36930

      AND scadenziario.esito IS NULL
      AND id_piano=23
      AND id_veterinario_programmato=321

it says:

cx_Oracle.DatabaseError: ORA-04091: table JOSE.SCADENZIARIO is mutating, trigger/function may not see it
ORA-06512: at "JOSE.SCADENZIARIO_UPDATE_TRIGGER", line 4 ORA-04088: error during execution of trigger 'JOSE.SCADENZIARIO_UPDATE_TRIGGER' I can't see what's wrong with it.
Could someone, please, get me some help?

Thank you in advance.

j

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