find_data() only works if model created with a formula

[Thomas J. Leeper]

Example:

library("MASS")
tr <- sample(1:50, 25)
train <- rbind(iris3[tr,,1], iris3[tr,,2], iris3[tr,,3])
test <- rbind(iris3[-tr,,1], iris3[-tr,,2], iris3[-tr,,3])
cl <- factor(c(rep("s",25), rep("c",25), rep("v",25)))
zq <- qda(train, cl)
find_data(zq)
## Error in terms.formula(formula, data = data) : 
##   argument is not a valid model