When CDMInstanceProxy::startedWaitingForKey dispatches the the main thread, it's possible the MediaPlayer is still alive, but the associated client has been destroyed/corrupted somehow.

void CDMInstanceProxy::startedWaitingForKey()                                                                                                                   │
   │270     {                                                                                                                                                               │
   │271         ASSERT(!isMainThread());                                                                                                                                    │
   │272         ASSERT(m_player);                                                                                                                                           │
   │273                                                                                                                                                                     │
   │274         bool wasWaitingForKey = m_numDecryptorsWaitingForKey > 0;                                                                                                   │
   │275         m_numDecryptorsWaitingForKey++;                                                                                                                             │
   │276                                                                                                                                                                     │
   │277         callOnMainThread([player = m_player, wasWaitingForKey] {                                                                                                    │
   │278             if (player && !wasWaitingForKey)                                                                                                                        │
  >│279                 player->waitingForKeyChanged();                                                                                                                     │
   │280         });                                                                                                                                                         │
   │281     }

The player pointer is fine, then we go to,

   │1324    void MediaPlayer::waitingForKeyChanged()                                                                                                                        │
   │1325    {                                                                                                                                                               │
  >│1326        client().mediaPlayerWaitingForKeyChanged();                                                                                                                 │
   │1327    } 

And hit a SIGSEGV. The crash must be happening on client(),

(gdb) p m_client
$4 = (WebCore::MediaPlayerClient *) 0xffff70b58d48105d

(gdb) p *m_client
Cannot access memory at address 0xffff70b58d48105d

That's a funny looking pointer address with the all the most-significant 1 bits. This must be some unpleasant JS GC interferance, I am unsure how this can be avoided from the CDMInstanceProxy POV.