code: public IssueCategory create() throws RedmineException {
          if (getProjectId() == null) {
              throw new IllegalArgumentException(
                      "IssueCategory must contain projectId");
          }
  
          return transport.addChildEntry(Project.class, getProjectId().toString(), this);
      }
docs: creates a new {@link IssueCategory} for the {@link Project} contained. <br>
  Pre-condition: the attribute {@link Project} for the {@link IssueCategory} must
  not be null!
  
  @return the new {@link IssueCategory} created by Redmine
  @throws IllegalArgumentException       thrown in case the category does not contain a project.
  @throws RedmineAuthenticationException thrown in case something went wrong while trying to login
  @throws RedmineException               thrown in case something went wrong in Redmine
  @throws NotFoundException              thrown in case an object can not be found