FEC-14226: protect against missing textTracks in fast entry change (#800

)

### Description of the Changes

Please add a detailed description of the change, whether it's an
enhancement or a bugfix.
If the PR is related to an open issue please link to it.

### CheckLists

- [x] changes have been done against master branch, and PR does not
conflict
- [-] new unit / functional tests have been added (whenever applicable)
- [x] test are passing in local environment
- [x] Travis tests are passing (or test results are not worse than on
master branch :))
- [-] Docs have been updated

src/player.ts

@@ -2491,7 +2491,7 @@ export default class Player extends FakeEventTarget {
    * @private
    */
   private _getTracksByType<T extends PKTextTrack | AudioTrack | VideoTrack | ImageTrack>(type: { new (...args: any[]): T }): T[] {
-    return this._tracks.reduce((arr: T[], track) => {
+    return this._tracks?.reduce((arr: T[], track) => {
       if (track instanceof type && track.available) {
         arr.push(track);
       }

src/track/external-captions-handler.ts

@@ -502,7 +502,7 @@ class ExternalCaptionsHandler extends FakeEventTarget {
    * @private
    */
   private _maybeSetExternalCueIndex(): boolean {
-    const textTrack = this._player._getTextTracks().find(track => track.active && track.external);
+    const textTrack = this._player._getTextTracks()?.find(track => track.active && track.external);
     if (textTrack && textTrack.external) {
       const cues = this._textTrackModel[textTrack.language] ? this._textTrackModel[textTrack.language].cues : [];
       let i = 0;