update subtitle.js

docs/uncompiled/artplayer/index.js

@@ -3732,6 +3732,7 @@ class Subtitle extends (0, _componentDefault.default) {
         art.on("video:timeupdate", ()=>{
             if (!this.url) return;
             const state = this.art.template.$video.webkitDisplayingFullscreen;
+            if (typeof state !== "boolean") return;
             if (state !== lastState) {
                 lastState = state;
                 this.createTrack(state ? "subtitles" : "metadata", this.url);