The exception mentions the 'environment' is null...
maybe instead of 'exec', try using processbuilder as in the below sample of server-side code running ffmpeg wrapped in a shellscript(pars_submit)... note builder.environment
        public String getFfmpeg(@QueryParam("infil1") String infil1, 
                @QueryParam("infil2") String infil2, @QueryParam("otfil") String otfil,
                @QueryParam("t") String time) {         
        String outfil = "dummy.mp4";
          List<String> command = new ArrayList<String>();
            command.add("vendor/bin/pars_submit");
            command.add(infil1);     
            command.add(infil2);
            command.add(otfil);
            command.add(time);
System.out.println("Starting process " +command.toString());
            ProcessBuilder builder = new ProcessBuilder(command);
            Map<String, String> environ = builder.environment();
//          for(Entry<String, String> entry : environ.entrySet()){
    //          System.out.println("ENV " +entry.getKey() + " " +entry.getValue());
      //      }
//          builder.redirectErrorStream(true);
            Process process = null;
            try {
                process = builder.start();
            InputStream is = process.getInputStream();
            InputStreamReader isr = new InputStreamReader(is);
            BufferedReader br = new BufferedReader(isr);
            String line;
            while ((line = br.readLine()) != null) {  
              //System.out.println(line);
                outfil=line;
            }
//          int exitVal = process.waitFor();
 //           System.out.println("Process exitValue: " + exitVal);
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }  
            finally {
                  if (process != null) {
                    process.destroy();
                    process = null;
                  }
                }           
            return outfil;
                }
IMO - android-ffmpeg is going to be more reliable using a full JNI interface and wrapper for the call to ffmpeg.main(). Look at all the git projects... search on 'android-ffmpeg' and look how they invoke ffmpeg. CLI is not used. 
pars_submit
#!/bin/bash
shopt -s globstar
uri=$1
filnam="${uri##*/}"
uri2=$2
filnam2="${uri2##*/}"
otfil=$3
time=$4
curl -#LO  $uri
curl -#LO  $uri2
ffmpeg -y -loop 1 -i "$filnam" -i "$filnam2" -t "$time" -r 1/2 -pass 1 -vcodec libx264 -b:v 200k -bt 50k  -an -f mp4 -strict -2 -passlogfile mydummy /dev/null
# echo "ffmpegP1 Exit status" $?
ffmpeg -y -loop 1 -i "$filnam" -i "$filnam2" -t "$time" -r 1/2 -pass 2 -vcodec libx264 -b:v 200k -bt 50k  -f mp4 -strict -2 -passlogfile mydummy -ar 44100 "$otfil" 
# echo "ffmpegp2 Exit status" $?
# last test
json=$(curl  -X POST  -H "X-Parse-Application-Id: 3KxPB" -H "X-Parse-REST-API-Key: kVl5Z0CX"  -H "Content-Type: video/mp4"  --data-binary @"$otfil" https://api.parse.com/1/files/"$otfil")
# echo "parse POST Exit status" $?
echo $json