How to represent a bi-directional list in db?

I am creating an application for a manufacturing scenario. To represent
stages in an assembly line, I wanted to create following table,

CREATE TABLE stages (
id SERIAL PRIMARY KEY,
name VARCHAR(80) NOT NULL,
created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
prev_stage_id SERIAL REFERENCES stages NULL,
next_stage_id SERIAL REFERENCES stages NULL,
process_id SERIAL REFERENCES processes NOT NULL
);

But it:

Failed with: conflicting NULL/NOT NULL declarations for column
"prev_stage_id" of table "stages"

Is it not possible to create "nullable" self referencing foreign keys?

--
Pankaj Jangid