Update isPrepared to be consistent with DisplaySearch's isPrepared

ext/bg/js/backend.js

@@ -67,8 +67,6 @@ class Backend {
             url: window.location.href
         };
 
-        this.isPrepared = false;
-
         this.clipboardPasteTarget = document.querySelector('#clipboard-paste-target');
 
         this.popupWindow = null;
@@ -77,6 +75,8 @@ class Backend {
 
         this.messageToken = yomichan.generateId(16);
 
+        this._isPrepared = false;
+
         this._messageHandlers = new Map([
             ['yomichanCoreReady', {handler: this._onApiYomichanCoreReady.bind(this), async: false}],
             ['optionsSchemaGet', {handler: this._onApiOptionsSchemaGet.bind(this), async: false}],
@@ -144,8 +144,6 @@ class Backend {
         }
         chrome.runtime.onMessage.addListener(this.onMessage.bind(this));
 
-        this.isPrepared = true;
-
         const options = this.getOptions(this.optionsContext);
         if (options.general.showGuide) {
             chrome.tabs.create({url: chrome.runtime.getURL('/bg/guide.html')});
@@ -156,6 +154,12 @@ class Backend {
         this._sendMessageAllTabs('backendPrepared');
         const callback = () => this.checkLastError(chrome.runtime.lastError);
         chrome.runtime.sendMessage({action: 'backendPrepared'}, callback);
+
+        this._isPrepared = true;
+    }
+
+    isPrepared() {
+        return this._isPrepared;
     }
 
     _sendMessageAllTabs(action, params={}) {

ext/bg/js/util.js

@@ -60,7 +60,7 @@ function utilBackgroundFunctionIsolate(func) {
 
 function utilBackend() {
     const backend = chrome.extension.getBackgroundPage().yomichanBackend;
-    if (!backend.isPrepared) {
+    if (!backend.isPrepared()) {
         throw new Error('Backend not ready yet');
     }
     return backend;