Referring to a data slot (Error in object of a basic class ("NULL") with no slots)

Question

I am running a simulation and need to refer to a matrix of parameters from a large object. Here is a snippet of the object structure itself:

Formal class 'mi' [package "mi"] with 3 slots
  ..@ call       : language .local(y = y, n.chains = ..2, max.minutes = 20000)
  ..@ data       :List of 100
  .. ..$ chain:1  :'data.frame':    10000 obs. of  76 variables:
Formal class 'missing_data.frame' [package "mi"] with 17 slots
  .. .. .. ..@ .Data       : list()
  .. .. .. ..@ variables   :List of 76
  .. .. .. .. ..$ y.obs.tx:Formal class 'binary' [package "mi"] with 27 slots
...

The parameter list I need can be referenced by:

mi.control.i@data$'chain:1'@variables$y.obs.tx@parameters[30,]

Since this is a simulation, I would like to grab the parameters from each chain and join them together in a matrix. I have this:

tf <- vector("list", numberofchains)
for (j in 1:numberofchains){
  s <- paste("'chain:",j,"'" ,sep="")
  tf[[j]] =  mi.control.i@data$s@variables$y.obs.tx@parameters[30,]
}

Within the loop, however, the reference to the object (tf[[j]]) does not work. I get an error that reads

Error: trying to get slot "variables" from an object of a basic class ("NULL") with no slots

Any ideas?