get_variance() returns NULL when modeling variance in brms

[matschmitz]

The get_variance() function returns NULL when used with brms models that include a formula for modeling variance (e.g., sigma ~ cyl).

library(brms)
library(insight) #v0. 20.4

m <- brm(bf(mpg ~ hp + (1 | cyl), sigma ~ cyl),
         data = mtcars, seed = 123)
get_variance(m)  # Unexpectedly returns NULL