It is an affirmative defense to prosecution under sections 28-520 and 28-521 that:
(1) A building or occupied structure involved in an offense under section 28-520 was abandoned; or
(2) The premises were at the time open to members of the public and the actor complied with all lawful conditions imposed on access to or remaining in the premises; or
(3) The actor reasonably believed that the owner of the premises or other person empowered to license access thereto would have licensed him to enter or remain; or
(4) The actor was in the process of navigating or attempting to navigate with a nonpowered vessel any stream or river in this state and found it necessary to portage or otherwise transport the vessel around any fence or obstructions in such stream or river.