And said party of the second part further covenants and agrees, that he will not assign this lease or under let or sublet said premises, or any part thereof, to any person or persons whomsoever, without first obtaining the written consent of said party of the first part; and notice is hereby given that any assignment of this lease or underletting of said premises not in strict conformity to this provision shall be absolutely null and void: Provided, however, that the consent of said party of the first part shall not be unreasonably nor capriciously withheld.