(1) A contract to make a will or devise or not to revoke a will or devise or to die intestate, if executed after July 1, 1975, may be established only by:
(a) provisions of a will stating material provisions of the contract;
(b) an express reference in a will to a contract and extrinsic evidence proving the terms of the contract; or
(c) a writing signed by the decedent evidencing the contract.
(2) The execution of a joint will or mutual wills does not create a presumption of a contract not to revoke the will or wills.