Hi @Andrew_Chen, welcome to the forum! 
The value of "{{ item }}" within an iteration would reference the object from the current iteration, which works if you are looking at a container inside of a List View component. However, outside of it, within the scope of the side panel, you do not have access to what is being clicked on the List View.
Here is an example of achieving this functionality with localStorage. @Kabirdas uploaded a JSON file you can use to create an app with and get familiarized with the approach.
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