Procedure to establish contract to make will or devise or not to revoke will or devise. (1) A contract to make a will or devise, or not to revoke a will or devise, or to die intestate, executed after January 1, 1974, shall 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. [1973 c.506 §13]