Merge multiple lists with different element lengths into a data frame in a function

Question

I have a function to extract rules of Decision Tree

data(iris) 
names(iris)[names(iris) == "Sepal.Length"] <- "SL"
names(iris)[names(iris) == "Sepal.Width"] <- "SW"
names(iris)[names(iris) == "Petal.Length"] <- "PL"
names(iris)[names(iris) == "Petal.Width"] <- "PW"
library(rpart)
set.seed(10)
pohon <- rpart(Species ~ ., iris,
               method='class',
               control=rpart.control(minsplit = 5, cp=0))

library(reshape)
rules.rpart <- function(model){
  if (!inherits(model, "rpart")) stop("Not a legitimate rpart tree")
  frm     <- model$frame 
  names   <- row.names(frm) 
  ylevels <- attr(model, "ylevels") 
  ds.size <- model$frame[1,]$n
  for (i in 1:nrow(frm)) 
  { 
    if (frm[i,1] == "<leaf>") 
    {
      prediksi=ylevels[frm[i,]$yval] 
      pth <- path.rpart(model, nodes=as.numeric(names[i]), print.it=F)
      urutan=unlist(pth)[-1]
      ur <- pth[-1]
      a=paste(urutan)
      a1=t(data.frame(a))
      df=data.frame(prediksi,a1)
      print(bind_rows(list(df))) 
    }}}
rules.rpart(pohon)

bb <- rules.rpart(pohon)
bb

My questions is:

1. How can I convert the output into a single data frame from several lists (different number of lists) with different element lengths?

2. Why I can't define the output into an object named "bb"? why does bb become NULL when called?