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, can be established only by: 1. Provisions of a will stating material provisions of the contract;
2. An express reference in a will to a contract and extrinsic evidence proving the terms of the contract; or 3. A writing signed by the decedent evidencing the contract. The execution of a joint will or mutual wills does not create a presumption of a contract not to revoke the will or wills.