title: FFmpeg.execute.FFmpegExecuteResponseHandler on a null object reference
body: <p>My Code:</p>
  
  <pre><code>submit.setOnClickListener(new View.OnClickListener() {
         @Override
         public void onClick(View v) {
             execFFmpegBinary( " -i" + newFile.getAbsolutePath() + " -codec:v libx264 -profile:v high -preset slow -b:v 500k -maxrate 500k -bufsize 1000k -vf scale=-1:480 -threads 0 -pass 2 -codec:a libfdk_aac -b:a 128k -f mp4 " + dest1.getAbsolutePath());
  
         }
     });
  
  
  public void execFFmpegBinary(final String command) {
      try {
          Log.d("path....",command);
          ffmpeg.execute( command, new ExecuteBinaryResponseHandler() {
              @Override
              public void onFailure(String s) {
                  Log.d(TAG, "FAILED with output : " + s);
              }
  
              @Override
              public void onSuccess(String s) {
                  Log.d(TAG, "SUCCESS with output : "+s);
              }
  
              @Override
              public void onProgress(String s) {
                  Log.d(TAG, "Started command : ffmpeg "+command);
                  Log.d(TAG, "progress : " + s);
              }
  
              @Override
              public void onStart() {
                  Log.d(TAG, "Started command : ffmpeg " + command);
                  progressDialog.setMessage("Processing...");
                  progressDialog.show();
              }
  
              @Override
              public void onFinish() {
                  Log.d(TAG, "Finished command : ffmpeg " + command);
                  progressDialog.dismiss();
              }
          });
      } catch (FFmpegCommandAlreadyRunningException e) {
          // do nothing for now
      }
  }
  </code></pre>
  
  <p>Show this error:  </p>
  
  <pre><code>FATAL EXCEPTION: main
  Process: com.example.mypc.videocut, PID: 26910
  java.lang.NullPointerException: Attempt to invoke virtual method 'void com.github.hiteshsondhi88.libffmpeg.FFmpeg.execute(java.lang.String, com.github.hiteshsondhi88.libffmpeg.FFmpegExecuteResponseHandler)' on a null object reference
  </code></pre>