Javafx Adding slide to powerpoint while still open

I want to be able to continuously add slides to a powerpoint while it is still open and not having to close it every time. I know its the FileOutputStream I just cant find a way to work around it.

// get snapshot of node scene and save to image
  WritableImage snapshot = node.getScene().snapshot(null);

  // create new slide show
  File isFile = new File(pptName);

  if(isFile.exists()){
    ppt = new XMLSlideShow(new FileInputStream(pptName));

  } else{
    ppt = new XMLSlideShow();
  }

  // set the page size to match our screen size
  ppt.setPageSize(Toolkit.getDefaultToolkit().getScreenSize());

  // create slide
  XSLFSlide slide = ppt.createSlide();

  try {

     // save temp image to disk
     saveImage(imgName, snapshot);

     // read the image file
     pictureData = IOUtils.toByteArray(new FileInputStream(imgName));

     // add the image to PPT
     int idx = ppt.addPicture(pictureData,
           XSLFPictureData.PICTURE_TYPE_PNG);
     slide.createPicture(idx);

     // save the PPT          
     pptFile = new FileOutputStream(pptName, true);

     ppt.write(pptFile);

     pptFile.close();

     LOGGER.info("Saved to Powerpoint..");

     // delete the temp image
     deleteImage(imgName);

     // open the PPT file in Powerpoint
     openFileInApplication(pptName);

Here is a quick snippet. Once it reaches the FileOutputStream IF the powerpoint is still open it will throw a "file not found error". If I close the powerpoint then it'll add to the powerpoint and reopen it.