`as.data.frame.markov_msm()` stops with error (corner case)

Thank you, Mark, for this great R package!

This bug bites when one tries to convert to data frame a markov_msm object returned by the standardise() function, but only if newdata passed to standardise() has 1 column.

Error in `[.default`(x$newdata, as.numeric(out$obs), , drop = FALSE) : 
  incorrect number of dimensions

The error seems to be due to the class of the object x$newdata <- x$newdata[0, ] returned by standardise(), which for a 1-column data frame is:

> df <- data.frame(rx = levels(survival::colon$rx))
> df[0, ] |> class() 
[1] "character"

while for data frames with >1 column, is:

> df <- data.frame(rx = levels(survival::colon$rx),
+                  age = c(65, 70, 75))
> df[0, ] |> class()
[1] "data.frame"

Reproducible example:

two_states <- function(model, ...) {
  transmat = matrix(c(NA,1,NA,NA),2,2,byrow=TRUE)
  rownames(transmat) <- colnames(transmat) <- c("Initial","Final")
  rstpm2::markov_msm(list(model), ..., trans = transmat)
}

## ERROR
death = gsm(Surv(time,status)~factor(rx), 
            data=survival::colon, subset=(etype==2), df=3)
ts = two_states(death, newdata=data.frame(rx = levels(survival::colon$rx)), 
                t = seq(0,2500, length = 51))
sts <- standardise(ts)
as.data.frame(sts)

## OK
death = gsm(Surv(time,status)~factor(rx) + age, 
            data=survival::colon, subset=(etype==2), df=3)
ts = two_states(death, newdata=data.frame(rx="Obs",
                                          age = c(65, 70, 75)), 
                t = seq(0,2500, length = 51))
sts <- standardise(ts)
as.data.frame(sts)

rstpm2 version 1.5.3, R version 4.2.0

mclements / rstpm2

`as.data.frame.markov_msm()` stops with error (corner case) #28