Error for `as.matrix()` applied to GAMM projection

Applying as.matrix() to a GAMM projection does not work:

# Source: Example section of `?rstanarm::stan_gamm4`.
library(projpred)
library(rstanarm)
options(mc.cores = parallel::detectCores(logical = FALSE))
dat <- mgcv::gamSim(1, n = 400, scale = 2) ## simulate 4 term additive truth
## Now add 20 level random effect`fac'...
dat$fac <- fac <- as.factor(sample(1:20, 400, replace = TRUE))
dat$y <- dat$y + model.matrix(~ fac - 1) %*% rnorm(20) * .5
br <- stan_gamm4(y ~ s(x0) + x1 + s(x2), data = dat, random = ~ (1 | fac),
                 chains = 1,
                 iter = 500, # for example speed
                 seed = 1140350788)

myproj <- project(br,
                  solution_terms = c("x1", "s(x2)", "(1 | fac)"),
                  nclusters = 2)
myprjmat <- as.matrix(myproj)

On branch develop (for example), that last line raises the following error:

Error in names(population_effects) <- replace_intercept_name(names(population_effects)) :
  attempt to set an attribute on NULL

The issue is probably that as.matrix.lm() (which is used for submodels of class "gamm4") uses coef() which in turn is NULL for objects of class "gamm4":

coef(myproj$sub_fit[[1]])
## --> NULL

So for submodels of class "gamm4", the as.matrix.lm() method probably can't be used.

Reproduce the example from ?rstanarm::stan_gamm4 and start at as.matrix(myproj), tracing into as.matrix.lm and coef(myproj$sub_fit[[1]]). Compare the GAMM submodel path with the failing names(population_effects) assignment. Done means as.matrix(myproj) completes for the shown GAMM projection without the NULL-related error.