R text mining: error in extracting bigrams from corpus during tokenization step

Question

Fetching data from url:

suppressMessages(library(readr))
suppressMessages(library(RCurl))


amazon_url <- getURL('http://s3.amazonaws.com/assets.datacamp.com/production/course_935/datasets/500_amzn.csv',
      ssl.verifyhost=FALSE, ssl.verifypeer=FALSE)
amazon <- read.csv(textConnection(amazon_url), header = TRUE)     

Create amzn_cons:

amazon_cons <- amazon$cons

Build cleaning function based qdap package for text organization:

suppressWarnings(library(qdap))
qdap_clean <- function(x) {
x <- replace_abbreviation(x)
x <- replace_contraction(x)
x <- replace_number(x)
x <- replace_ordinal(x)
x <- replace_symbol(x)
x <- tolower(x)
return(x)
}

Build cleaning function based on tm package for text organization:

suppressWarnings(library(tm))
tm_clean <- function(corpus) {
      corpus<- tm_map(corpus, removePunctuation)
      corpus <- tm_map(corpus, stripWhitespace)
      corpus <- tm_map(corpus, removeWords,
      c(stopwords("en"), "Amazon","company"))
      return(corpus)
}

Word cleaning:

amzn_cons <- qdap_clean(amazon_cons)
amzn_cons <- VCorpus(VectorSource(amzn_cons))
amzn_cons_corp <- tm_clean(amzn_cons)

Build custom function to extract bigram features:

    suppressWarnings(library(RWeka))
    tokenizer <- function(x) 
    NGramTokenizer(x, Weka_control(min = 2, max = 2))

Apply tokenization function to get bigrams words:

  amzn_c_tdm <- TermDocumentMatrix(
  amzn_cons_corp,control = list(tokenize = tokenizer) )

This results in the following error:

Error in .jcall("RWekaInterfaces", "[S", "tokenize", .jcast(tokenizer,  : 
  java.lang.NullPointerException

How to solve this error?