Assignability of Purchase & Sale contract in Kansas - agreed to by all parties?

[David Heyburn]

I'm reviewing a KCCAR - Kansas Purchase & Sales agreement and when reviewing for assignability, it has the clause: 

15. ENTIRE AGREEMENT AND MANNER OF MODIFICATIONS. This Contract and all attachments
constitute the complete agreement of the parties concerning the Property, supersede all previous
agreements, and may be modified or assigned only by a written agreement signed by all parties.

Is this a law in Kansas? Or does the contract simply need to be modified, or have a different contract that permits assignability?

Thanks

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