Fix a mini sidebar bug when embedding admin-lte into iframe (#4799)

When embedding whole admin-lte page into a iframe, mini sidebar will display without content.

build/js/IFrame.js

@@ -272,7 +272,7 @@ class IFrame {
   }
 
   _initFrameElement() {
-    if (window.frameElement && this._config.autoIframeMode) {
+    if (window.frameElement && this._config?.autoIframeMode) {
       const $body = $('body')
       $body.addClass(CLASS_NAME_IFRAME_MODE)