Error in If else statement in R for NULL check

Can anyone please help me to solve the error. I want to check in IF statement that, if there is no Null then insert the value.

 while (i < 33 )
{
 library(jsonlite)
 library(httr)
 get_reply2 <- my_get(staypoints$Stayplon[i],staypoints$staypLat[i], 2)
 get_replyFF2 <- append(get_replyFF2, get_reply2$value$id) 
 get_replyFG2 <- append(get_replyFG2, get_reply2$value$title) 
  if (get_replyFF2[i] != 0)
  {
  LONGG <- append(LONGG, staypoints$Stayplon[i])
  LATT <- append(LATT, staypoints$Stayplon[i])
  }

   i <- i + 1
 };

 Error: Error in if (get_replyFF2[i] != 0) { : 
 missing value where TRUE/FALSE needed