Groovy connector file reading script returns null

Hello, I created a groovy script connector in order to read a CSV file.
With a local file, I can get the result without error; however, I could not read the CSV file with document url as follows. The url is correct and tested already.
It returns null.
Would you someone know the reason why? The filename in the script is not valid?

Any help would be appreciated.
Thanks in advanced.

//def fileName= "D:/BonitaAddOns/csv/testFile.csv"
def fileName= "/bonita/portal/"+ csvObjVar.docUrl

    File file= new File(fileName)
    List<List<String>> lines = new ArrayList<>()
    Scanner inputStream

    try{
        inputStream = new Scanner(file)

        while(inputStream.hasNext()){
            String line= inputStream.next()
            String[] values = line.split(",") 
            lines.add(Arrays.asList(values))

        }

        inputStream.close()
        return lines


    }catch (FileNotFoundException e) {
        e.printStackTrace()
    }