Every owner of, and every person in control of, any estate hereafter maintaining a sidewalk vault, coal hole, manhole or any other excavation, or any post, pole, sign, awning, wire, pipe, conduit or other structure in, under, over or upon any street which is adjacent to or a part of his or her estate, shall do so only on the condition that such maintenance is considered an agreement on his or her part with the City to keep the same and the covers thereof, and any gas and electric boxes and tubes thereon, in good repair and condition at all times during his or her ownership or control thereof and to indemnify and save harmless the City against all damages or actions at law that may arise or be brought by reason of such excavation or structure being under, over, in or upon the street or being unfastened, out of repair or defective during such ownership or control.