TY JOUR TI BREACHES IN UNILATERAL TERMINATION OF AN OBLIGATION: CONSEQUENCES AND POSSIBILITIES OF CONVALIDATION KW contract KW unilateral refusal KW performance of obligations KW payment KW secondary right KW juridical fact KW termination KW termination of legal relationship KW estoppel JO Advances in Law Studies AU Grigorev, O.I. PY 2026 IS 14 PB Publishing Center RIOR