Can't read webvtt in webm file

Hi,
I have a webm file. It has an subtitle stream by webvtt format. But Exoplayer can't find it.

#### THE VIDEO INFO:
```
Input #0, matroska,webm, from 'a1.webm':
Metadata:
ENCODER : Lavf59.27.100
Duration: 00:01:59.69, start: -0.006000, bitrate: 2762 kb/s
Stream #0:0: Video: vp9 (Profile 0), yuv420p(tv, progressive), 1920x1080, SAR 1:1 DAR 16:9, 25 fps, 25 tbr, 1k tbn (default)
Metadata:
BPS-eng : 8169907
DURATION-eng : 00:41:52.280000000
NUMBER_OF_FRAMES-eng: 62807
NUMBER_OF_BYTES-eng: 2565636839
_STATISTICS_WRITING_APP-eng: mkvmerge v25.0.0 ('Prog Noir') 64-bit
_STATISTICS_WRITING_DATE_UTC-eng: 2018-09-07 00:24:14
_STATISTICS_TAGS-eng: BPS DURATION NUMBER_OF_FRAMES NUMBER_OF_BYTES
ENCODER : Lavc59.37.100 libvpx-vp9
DURATION : 00:01:59.120000000
Stream #0:1(eng): Audio: opus, 48000 Hz, stereo, fltp (default)
Metadata:
BPS-eng : 640000
DURATION-eng : 00:41:52.320000000
NUMBER_OF_FRAMES-eng: 78510
NUMBER_OF_BYTES-eng: 200985600
_STATISTICS_WRITING_APP-eng: mkvmerge v25.0.0 ('Prog Noir') 64-bit
_STATISTICS_WRITING_DATE_UTC-eng: 2018-09-07 00:24:14
_STATISTICS_TAGS-eng: BPS DURATION NUMBER_OF_FRAMES NUMBER_OF_BYTES
ENCODER : Lavc59.37.100 libopus
DURATION : 00:01:59.688000000
Stream #0:2(eng): Subtitle: webvtt (default)
Metadata:
title : English
ENCODER : Lavc59.37.100 webvtt
DURATION : 00:01:55.514000000
```

#### THE EXOPLAYER LOG:

```
I/ExoPlayerImpl: Init cf736eb [ExoPlayerLib/2.18.4] [redfin, Pixel 5, Google, 33]
D/EventLogger: playWhenReady [eventTime=0.00, mediaPos=0.00, window=0, true, USER_REQUEST]
D/EventLogger: surfaceSize [eventTime=0.00, mediaPos=0.00, window=0, 0, 0]
D/EventLogger: timeline [eventTime=0.00, mediaPos=0.00, window=0, periodCount=1, windowCount=1, reason=PLAYLIST_CHANGED
D/EventLogger: period [?]
D/EventLogger: window [?, seekable=false, dynamic=true]
D/EventLogger: ]
D/EventLogger: mediaItem [eventTime=0.00, mediaPos=0.00, window=0, reason=PLAYLIST_CHANGED]
D/EventLogger: state [eventTime=0.00, mediaPos=0.00, window=0, BUFFERING]
E/OpenGLRenderer: Unable to match the desired swap behavior.
D/EventLogger: surfaceSize [eventTime=0.03, mediaPos=0.00, window=0, 1080, 2138]
E/SurfaceSyncer: Failed to find sync for id=0
W/Parcel: Expecting binder but got null!
D/EventLogger: loading [eventTime=0.03, mediaPos=0.00, window=0, period=0, true]
D/EventLogger: timeline [eventTime=0.03, mediaPos=0.00, window=0, period=0, periodCount=1, windowCount=1, reason=SOURCE_UPDATE
D/EventLogger: period [?]
D/EventLogger: window [?, seekable=false, dynamic=false]
D/EventLogger: ]
D/EventLogger: timeline [eventTime=0.33, mediaPos=0.00, window=0, period=0, periodCount=1, windowCount=1, reason=SOURCE_UPDATE
D/EventLogger: period [2512.33]
D/EventLogger: window [2512.33, seekable=true, dynamic=false]
D/EventLogger: ]
D/EventLogger: videoEnabled [eventTime=0.33, mediaPos=0.00, window=0, period=0]
D/EventLogger: audioEnabled [eventTime=0.33, mediaPos=0.00, window=0, period=0]
D/EventLogger: tracks [eventTime=0.33, mediaPos=0.00, window=0, period=0
D/EventLogger: group [
D/EventLogger: [X] Track:0, id=1, mimeType=video/x-vnd.on2.vp9, res=1920x1080, language=und, selectionFlags=[default], supported=YES
D/EventLogger: ]
D/EventLogger: group [
D/EventLogger: [X] Track:0, id=2, mimeType=audio/opus, channels=2, sample_rate=48000, language=en, selectionFlags=[default], supported=YES
D/EventLogger: ]
D/EventLogger: ]
I/DMCodecAdapterFactory: Creating an asynchronous MediaCodec adapter for track type video
D/EventLogger: downstreamFormat [eventTime=0.33, mediaPos=0.00, window=0, period=0, id=1, mimeType=video/x-vnd.on2.vp9, res=1920x1080, language=und, selectionFlags=[default]]
D/CCodec: allocate(c2.qti.vp9.decoder)
I/CCodec: setting up 'default' as default (vendor) store
I/CCodec: Created component [c2.qti.vp9.decoder]
D/CCodecConfig: read media type: video/x-vnd.on2.vp9
```

Exoplayer Version: 2.18.3
Devices: All

Thanks!