Neb. Rev. Stat. § 30-3827.01: Contracts concerning trusts.

A contract to make a trust, or not to revoke a trust, if executed on or after January 1, 1977, shall only be established by: (1) Provisions of a will or a trust stating the material provisions of the contract; (2) an express reference in a will or trust 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 trust does not create a presumption of a contract not to revoke the trust.