Adding two lists

[KaleBosRatjes]

So I'm trying to add two lists together with integers in the lists. In both lists there are strings with "null". I want to add the two lists together, but i want the null values of list 1 to be filled with the values that aren't "null" in list 2.

list1 = [5,6,'null',1,'null']
list2 = ['null','null',3,'null',8]

prefered_output = [5,6,3,1,8]

I hope you guys can help :)