Can only see page content after refreshing the page

HI, I am new to Frontity and following the tutorial to create a new theme. Until this step everything went fine(Display the post content - Frontity Tutorial).

While adding the code to show page content by placing this (const post = state.source[data.type][]) I need to refresh the page for it to load the content, same when i click on back browser button. It is just showing a white blank page.

Console gives the attached error.! Screen Shot 2022-02-11 at 9.59.33 pm|690x145