Q_t = array(NA, c(k, k, t0))
    B_t = array(NA, c(k, k*nlag, t0))
    pb = progress_bar$new(total= t0)
    for (i in 1:t0) {
      fit = var_model(x[i:(i+window.size-1),], configuration=configuration)
      B_t[,,i] = fit$B
      Q_t[,,i] = fit$Q
      pb$tick()
    }
Error in progress_bar$new(total = t0) : attempt to apply non-function

请问各位大神这个错误怎么修改呀？