THETA model can not using bootstrap #428

deaths |> model(theta = THETA(sqrt(value))) |> forecast(h = '4 years')

# A fable: 48 x 4 [1M]
# Key:     .model [1]
   .model    index          value  .mean
   <chr>     <mth>         <dist>  <dbl>
 1 theta  1979 Jan  t(N(91, 1.9))  8268.
 2 theta  1979 Feb  t(N(87, 2.5))  7504.
 3 theta  1979 Mar  t(N(91, 3.1))  8320.
 4 theta  1979 Apr  t(N(92, 3.7))  8552.
 5 theta  1979 May  t(N(97, 4.2))  9447.
 6 theta  1979 Jun  t(N(99, 4.8))  9869.
 7 theta  1979 Jul t(N(104, 5.4)) 10819.
 8 theta  1979 Aug   t(N(100, 6)) 10092.
 9 theta  1979 Sep  t(N(95, 6.5))  8947.
10 theta  1979 Oct  t(N(97, 7.1))  9333.
# ℹ 38 more rows
# ℹ Use `print(n = ...)` to see more rows

deaths |> model(theta = THETA(sqrt(value))) |> forecast(h = '4 years', bootstrap = TRUE, times = 100)

Error in `mutate()`:
ℹ In argument: `theta = (function (object, ...) ...`.
Caused by error in `UseMethod()`:
! no applicable method for 'generate' applied to an object of class "fable_theta"
Run `rlang::last_trace()` to see where the error occurred.
> rlang::last_trace()
<error/dplyr:::mutate_error>
Error in `mutate()`:
ℹ In argument: `theta = (function (object, ...) ...`.
Caused by error in `UseMethod()`:
! no applicable method for 'generate' applied to an object of class "fable_theta"
---
Backtrace:
     ▆
  1. ├─generics::forecast(...)
  2. ├─fabletools:::forecast.mdl_df(...)
  3. │ └─dplyr::mutate_at(...)
  4. │   ├─dplyr::mutate(.tbl, !!!funs)
  5. │   └─dplyr:::mutate.data.frame(.tbl, !!!funs)
  6. │     └─dplyr:::mutate_cols(.data, dplyr_quosures(...), by)
  7. │       ├─base::withCallingHandlers(...)
  8. │       └─dplyr:::mutate_col(dots[[i]], data, mask, new_columns)
  9. │         └─mask$eval_all_mutate(quo)
 10. │           └─dplyr (local) eval()
 11. ├─generics (local) `<fn>`(...)
 12. └─fabletools:::forecast.lst_mdl(...)
 13.   └─fabletools:::mapply_maybe_parallel(...)
 14.     └─base::mapply(FUN = .fp, ..., MoreArgs = MoreArgs, SIMPLIFY = SIMPLIFY)
 15.       └─fabletools (local) `<fn>`(...)
 16.         ├─generics (local) .f(...)
 17.         └─fabletools:::forecast.mdl_ts(...)
 18.           ├─generics::generate(...)
 19.           └─fabletools:::generate.mdl_ts(...)
 20.             └─generics::generate(...)
Run rlang::last_trace(drop = FALSE) to see 3 hidden frames.