docs: protected InputStream getDefaultDiagram(String diagramImageFileName) {
          String imageFileName = diagramImageFileName != null ?
                  diagramImageFileName :
                  getDefaultDiagramImageFileName();
          InputStream imageStream = getClass().getResourceAsStream(imageFileName);
          if (imageStream == null) {
              throw new ActivitiImageException("Error occurred while getting default diagram image from file: " + imageFileName);
          }
          return imageStream;
      }