Sums due for rent on leases under seal or otherwise and claims for damages to premises rented may be recovered in an action, specifying the items and amount claimed, but no action shall be maintained for any sum or sums claimed to be due for rental or for any claim for damages for the breach of any of the conditions claimed to be broken on the part of the lessee, his legal representatives, assigns or tenant, contained in a lease or written agreement to hire or occupy any building, buildings or part of a building, during a period when such building, buildings or part of a building, which the lessee, his assigns, legal representatives or tenant may occupy or have a right to occupy, shall have been destroyed or damaged by fire or other unavoidable casualty so that the same shall be thereby rendered unfit for use or habitation; provided that nothing herein shall render invalid or unenforceable an agreement contained in a lease of any building, buildings, or part of a building used primarily for other than residential purposes or in the case of any lease securing obligations guaranteed by the Maine Guaranty Authority or in any written instrument to pay the rental stipulated in said lease or agreement or any portion of such rental during a period when the building, buildings or part of a building described therein shall have been destroyed or damaged by fire or other unavoidable casualty so that the same shall be rendered unfit for use or habitation, in whole or in part. [PL 1973, c. 633, §21 (AMD).]