Type 'null' is not assignable to type 'string'

[ayinlaabel]

error TS2345: Argument of type 'string | null' is not assignable to parameter of type 'string'.
Type 'null' is not assignable to type 'string'.

23 return this.setSession(res.body._id, res.headers.get('x-access-token'), res.headers.get('x-refresh-token'));