don't assume the call handler is always set in device message handler

src/matrix/DeviceMessageHandler.js

@@ -74,6 +74,7 @@ export class DeviceMessageHandler {
     }
 
     async afterSyncCompleted(decryptionResults, deviceTracker, hsApi, log) {
+        if (this._callHandler) {
             // if we don't have a device, we need to fetch the device keys the message claims
             // and check the keys, and we should only do network requests during
             // sync processing in the afterSyncCompleted step.
@@ -100,6 +101,7 @@ export class DeviceMessageHandler {
             }
         }
     }
+}
 
 class SyncPreparation {
     constructor(olmDecryptChanges, newRoomKeys) {