elements in my components not displaying! :O

i am doing my capstone project on my udacity front end nanodegree, it is written in react, includes a neighourhood map (google API), and many parent/child & sibling component interactions.
the problem i am having is that, after i added functionality to connect two of the elements in a component (namely, an item in a mapped array with a list of locations has to trigger something on a google map marker (marker comes from its api)). when i added this, the list stopped displaying and, needless to say, there is no functionality. i am at a loss, almost 99.9% positive it is some error on my side but cannot see it/understand it because it is only my 2nd ever react project, and cannot find anyone available to help me rn.
So, here i am. found this resource on the reactjs.org documentation and giving it a try. bare with me as it is my first post ever, surely you'll want to see the code but idk which part and i didnt do it via github, it is local on my computer. i can upload it if necessary OR (better and quicker yet) post it wherever.

thank you so much for finding my bottle in the ocean :D & regards.