sqlplus trigger error

-----------------------------------------------------------------------
--  USERS
-----------------------------------------------------------------------
CREATE TABLE users
(
    id NUMBER(10) PRIMARY KEY NOT NULL,
    email CHAR(100) UNIQUE NOT NULL,
    password CHAR(60) NOT NULL,
    first_name CHAR(30) NOT NULL,
    last_name CHAR(30) NOT NULL,
    birth_date DATE NULL,
    gender CHAR(10) NULL,
    joined TIMESTAMP DEFAULT CURRENT_TIMESTAMP NOT NULL,
    CONSTRAINT gender_check CHECK (UPPER(gender) IN ('M', 'F'))
);

-- Auto increment for users.id
DROP SEQUENCE users_sequence;
CREATE SEQUENCE users_sequence START WITH 1 INCREMENT BY 1;
CREATE TRIGGER users_before_insert
BEFORE INSERT
    ON users REFERENCING NEW AS NEW
    FOR EACH ROW
BEGIN
    SELECT users_sequence.NEXTVAL INTO :NEW.ID FROM dual;
END;
/

--
-- POSTS
--
CREATE TABLE posts
(
    id NUMBER(10) PRIMARY KEY NOT NULL,
    user_id NUMBER(10) REFERENCES users(id) NOT NULL,
    created TIMESTAMP DEFAULT CURRENT_TIMESTAMP NOT NULL,
    content CHAR(1000) NOT NULL,
    likes NUMBER(10) DEFAULT 0 NOT NULL,
    privacy CHAR(10) DEFAULT 'public' NOT NULL,
    CONSTRAINT fk_posts FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE
);

-- Auto increment for posts.id
DROP SEQUENCE posts_sequence;
CREATE SEQUENCE posts_sequence START WITH 1 INCREMENT BY 1;
CREATE TRIGGER posts_before_insert
BEFORE INSERT
    ON posts REFERENCING NEW AS NEW
    FOR EACH ROW
BEGIN
    SELECT posts_sequence.NEXTVAL INTO :NEW.ID FROM dual;
END;
/

CREATE TRIGGER posts_before_delete
BEFORE DELETE
    ON posts
    FOR EACH ROW
BEGIN
    DELETE FROM post_likes WHERE post_id = :OLD.id;
END;
/

--
-- POST_LIKES
--
CREATE TABLE post_likes
(
    user_id NUMBER(10) NOT NULL REFERENCES users(id),
    post_id NUMBER(10) NOT NULL REFERENCES posts(id),
    PRIMARY KEY (user_id, post_id),
    --CONSTRAINT fk_post_likes1 FOREIGN KEY (post_id) REFERENCES posts(id) ON DELETE CASCADE,
    CONSTRAINT fk_post_likes2 FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE
);

-- Increments posts.likes
CREATE TRIGGER post_likes_after_insert
AFTER INSERT
    ON post_likes
    FOR EACH ROW

DECLARE
   n NUMBER(10);
BEGIN
    SELECT likes INTO n FROM posts WHERE id = :NEW.post_id;
    UPDATE posts SET likes = n+1 WHERE id = :NEW.post_id;
END;
/

-- Decrements posts.likes
CREATE TRIGGER post_likes_before_delete
BEFORE DELETE
    ON post_likes
    FOR EACH ROW

DECLARE
   n NUMBER(10);
BEGIN
    SELECT likes INTO n FROM posts WHERE id = :OLD.post_id;
    UPDATE posts SET likes = n-1 WHERE id = :OLD.post_id;
END;
/

-- Full error, EUSEBIU is the username that I'm logged in
-- ERROR at line 1:
-- ORA-04091: table EUSEBIU.POSTS is mutating, trigger/function may not see it
-- ORA-06512: at "EUSEBIU.POST_LIKES_BEFORE_DELETE", line 4
-- ORA-04088: error during execution of trigger 'EUSEBIU.POST_LIKES_BEFORE_DELETE'
-- ORA-06512: at "EUSEBIU.POSTS_BEFORE_DELETE", line 2
-- ORA-04088: error during execution of trigger 'EUSEBIU.POSTS_BEFORE_DELETE'