prepare Backend for browser_action

ext/bg/js/backend.js

@@ -278,6 +278,11 @@ class Backend {
     // Message handlers
 
     async _onApiYomichanCoreReady(_params, sender) {
+        // tab ID isn't set in background (e.g. browser_action)
+        if (typeof sender.tab === 'undefined') {
+            return chrome.runtime.sendMessage({action: 'backendPrepared'});
+        }
+
         const tabId = sender.tab.id;
         return new Promise((resolve) => {
             chrome.tabs.sendMessage(tabId, {action: 'backendPrepared'}, resolve);

ext/bg/js/context.js

@@ -48,7 +48,9 @@ function setupButtonEvents(selector, command, url) {
     }
 }
 
-window.addEventListener('DOMContentLoaded', () => {
+window.addEventListener('DOMContentLoaded', async () => {
+    await yomichan.prepare();
+
     showExtensionInfo();
 
     apiGetEnvironmentInfo().then(({browser}) => {