Access localStorage of chrome extension in background script

Question

I need to access the localStorage of a chrome extension in background.js to get values which have been set by the extension popup itself. The key-value pair is existing, I checked that by inspecting the extension popup.

However, reading from the localStorage by using the code below leads to an undefined result.

background.js:

chrome.runtime.onMessage.addListener(
    function(request, sender, sendResponse) {
        chrome.storage.local.get(['allowedSites'], function(result) {
            console.log('allowedSites: ', result.allowedSites);
        });
    }
);

The storage permission is set in v3 manifest. Any ideas what's wrong with this code?