R: Loop to check if file exists and take it to a list #fails

I have 735 folders and some of them will have a .grd file inside, others won't. After a lot of research here in stack, I came up with a code to check if the .grd file exists and, if it does, read the .grd into a list, if doesn't, skip it. But it isn't working... the code is resulting in a list with 735 objects, some of thm, empty. This is my code:

#the folder where all folders are
dir_maxent_spp<-"C:\\Users\\thai\\Documents\\ORCHIDACEAE\\Ecologicos\\w2\\SpDistModel\\Maxent_spp_20170815\\xm"

#list all folders within 
files <- list.files()

#create an empty list to receive the reads
grd_list<-list()

for (i in 1:length(files)){

#get into each folder
setwd(paste(dir_maxent_spp,"\\",files[i], sep="")) 

#set the name of the file I'm looking for
f <- paste0(paste(gsub('\\s+','_',files[i]),"_pred.grd", sep=""))

#check it's existence and it does exist, get it to the list
if (file.exists(f))
grd_list[[i]]<-stack(f)
}

Now, I do realize that the problem is this part: grd_list[[i]]<-stack(f) ... here I take the .grd into the i'th position of the list, right? But how do I skip a value for i without skipping the position in the list? I'm new to R, I'm sorry if the question is too naive