private Guid GetTabId()
{
var receivedCalls = _sendEndpoint.ReceivedCalls();
var sendCall = receivedCalls.Single(call => call.GetMethodInfo().Name == "Send");
var arguments = sendCall.GetArguments();
Assert.That(arguments[0] is IOpenTabCommand);
Assert.That(arguments[1] is CancellationToken);
var openTabCommand = (OpenTabCommand)arguments.First();
return(openTabCommand.AggregateId);
}