`rstanarm::loo` and `rstanarm::posterior_predict` return errors

Summary:

Problems with rstanarm:::loo.stanreg / rstanarm::loo from stan_betareg model fit

Description / reproducibility

> fit1 <- fit1 <- rstanarm::stan_betareg(
  formula= <formula>, 
  data= train, link = "logit", link.phi = "log", 
  prior= normal(), prior_intercept= normal(), prior_phi= exponential(),
  iter= iter, warmup= warmup, chains= chains, cores= chains)

> class(fit1)
[1] "stanreg" "betareg"
> loo(fit1, cores= 20)
Error in names(object[[n]])[1L:2L] <- c("mu", "phi") : 
  attempt to set an attribute on NULL
> rstanarm::loo(fit1, cores= 20)
Error in names(object[[n]]) <- `*vtmp*` : 
  attempt to set an attribute on NULL
> rstanarm:::loo.stanreg(fit1, cores= 20)
Error in names(object[[n]]) <- `*vtmp*` : 
  attempt to set an attribute on NULL
> loo(fit1)
Error in names(object[[n]]) <- `*vtmp*` : 
  attempt to set an attribute on NULL
> tt <- rstanarm::posterior_predict(fit1, draws= 1000, re.form= NULL, allow.new.levels= TRUE)
Error in names(object[[n]]) <- `*vtmp*` : 
  attempt to set an attribute on NULL

using example data:

see: https://cran.r-project.org/web/packages/rstanarm/vignettes/betareg.html

SEED <- 1234
set.seed(SEED)
eta <- c(1, -0.2)
gamma <- c(1.8, 0.4)
N <- 200
x <- rnorm(N, 2, 2)
z <- rnorm(N, 0, 2)
mu <- binomial(link = logit)$linkinv(eta[1] + eta[2]*x)
phi <- binomial(link = log)$linkinv(gamma[1] + gamma[2]*z)
y <- rbeta(N, mu * phi, (1 - mu) * phi)
dat <- data.frame(cbind(y, x, z))

fit1 <- stan_betareg(y ~ x | z, data = dat, link = "logit", link.phi = "log",
                     chains = 1L, cores = 1L, seed = 1L, iter = 100L)
loo(fit1)
Error in names(object[[n]]) <- `*vtmp*` : 
  attempt to set an attribute on NULL
tt <- rstanarm::posterior_predict(fit1, draws= 1000, re.form= NULL, allow.new.levels= TRUE)
Error in names(object[[n]]) <- `*vtmp*` : 
  attempt to set an attribute on NULL

session info

> sessionInfo()
R version 3.4.1 (2017-06-30)
Platform: x86_64-redhat-linux-gnu (64-bit)
Running under: CentOS Linux 7 (Core)

Matrix products: default
BLAS/LAPACK: /usr/local/lib/libtatlas.so

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C               LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8    
 [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8    LC_PAPER=en_US.UTF-8       LC_NAME=C                 
 [9] LC_ADDRESS=C               LC_TELEPHONE=C             LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] loo_1.1.0         rstanarm_2.17.2   Rcpp_0.12.13      betareg_3.2-0     fbmodels_0.1.3    ggplot2_2.2.1    
[7] data.table_1.11.0

loaded via a namespace (and not attached):
 [1] lattice_0.20-35    zoo_1.8-3          gtools_3.5.0       lmtest_0.9-36      assertthat_0.2.0   digest_0.6.12     
 [7] mime_0.5           R6_2.2.2           plyr_1.8.4         stats4_3.4.1       colourpicker_1.0   rlang_0.1.2       
[13] lazyeval_0.2.1     minqa_1.2.4        miniUI_0.1.1       nloptr_1.0.4       Matrix_1.2-10      DT_0.2            
[19] shinythemes_1.1.1  splines_3.4.1      shinyjs_1.0        lme4_1.1-14        stringr_1.2.0      htmlwidgets_0.9   
[25] igraph_1.1.2       munsell_0.4.3      shiny_1.0.5        compiler_3.4.1     httpuv_1.3.5       rstan_2.17.3      
[31] pkgconfig_2.0.1    base64enc_0.1-3    rstantools_1.4.0   htmltools_0.3.6    nnet_7.3-12        tibble_1.3.4      
[37] gridExtra_2.3      codetools_0.2-15   threejs_0.3.1      matrixStats_0.52.2 dplyr_0.7.4        MASS_7.3-47       
[43] grid_3.4.1         nlme_3.1-131       xtable_1.8-2       gtable_0.2.0       magrittr_1.5       StanHeaders_2.17.2
[49] scales_0.5.0       stringi_1.1.6      reshape2_1.4.2     flexmix_2.3-14     bindrcpp_0.2       dygraphs_1.1.1.4  
[55] xts_0.10-0         sandwich_2.4-0     Formula_1.2-3      tools_3.4.1        glue_1.2.0         shinystan_2.4.0   
[61] markdown_0.8       crosstalk_1.0.0    rsconnect_0.8.5    parallel_3.4.1     survival_2.41-3    yaml_2.1.19       
[67] inline_0.3.14      colorspace_1.3-2   bayesplot_1.4.0    bindr_0.1          modeltools_0.2-22 

Reproduce the failure with the linked beta-regression example and inspect the rstanarm:::loo.stanreg, loo, and posterior_predict entry points for stan_betareg objects. Trace the NULL value behind the reported naming error and verify that both loo(fit1) and posterior_predict(fit1, ...) complete successfully without the error.