message: set dispatcher ref to null so we don't double-unregister
diff: @@ -152,7 +152,10 @@ export default class CallHandler {
  if (cli) {
  cli.removeListener('Call.incoming', this.onCallIncoming);
  }
  - if (this.dispatcherRef !== null) dis.unregister(this.dispatcherRef);
  + if (this.dispatcherRef !== null) {
  + dis.unregister(this.dispatcherRef);
  + this.dispatcherRef = null;
  + }
  }
  private onCallIncoming = (call) => {
  