检测 inputVideo 是否存在

src/features/OnlineExam/Examing/FaceMotion/FaceMotion.vue

@@ -873,6 +873,7 @@ export default {
       await new Promise(resolve => {
         const interval = setInterval(() => {
           if (
+            videoEl &&
             videoEl.readyState === 4 &&
             faceapi.nets.tinyFaceDetector.params &&
             faceapi.nets.faceExpressionNet.params