Can't access popup DOM in chrome.runtime.onMessage

[leonma333]

in eighteenminus.js line 213 to send number of blocked images
chrome.runtime.sendMessage({event: 'number', number: imgBlocked}, function(response) {});

in popup.js line 61
chrome.runtime.onMessage.addListener(function(request, sender, sendResponse) { if (request.event == 'number') $('p.number').find('strong').text(request.number); });

inside onMessage, we can't access background content DOM or even use console. However, the data sent from content page is correct, and alert can be used.