docs: add some annotation

index.html

@@ -82,6 +82,7 @@
       </div>
     </div>
     <script>
+      // 此代码仅用于开发环境的友好提示，项目打包前请去掉这段js代码 This code is only used as a friendly reminder of the development environment, please remove this js code before packaging the project
       window.onload = function () {
         (function () {
           const ua = navigator.userAgent.toLowerCase();
@@ -97,16 +98,18 @@
 
           const el = document.querySelector("._develop");
 
-          if (browser && version >= 90) {
-            let success =
-              document.createTextNode("当前浏览器版本很适合开发！！！ 😃");
-            el.appendChild(success);
-          } else {
-            let warn = document.createTextNode(
-              "当前浏览器版本不适合开发，建议使用最新版本的谷歌或者火狐浏览器！！！😯"
-            );
-            el.appendChild(warn);
-            el.style.color = "red";
+          if (el) {
+            if (browser && version >= 90) {
+              let success =
+                document.createTextNode("当前浏览器版本很适合开发！！！ 😃");
+              el.appendChild(success);
+            } else {
+              let warn = document.createTextNode(
+                "当前浏览器版本不适合开发，建议使用最新版本的谷歌或者火狐浏览器！！！😯"
+              );
+              el.appendChild(warn);
+              el.style.color = "red";
+            }
           }
           return Sys;
         })();