code: public Extender build() throws KSIException {
          Util.notNull(extendingService, "KSI extending service");
          Util.notNull(publicationsHandler, "KSI publications handler");
          if (policy == null) {
              this.policy = ContextAwarePolicyAdapter.createInternalPolicy();
          }
          KSISignatureComponentFactory signatureComponentFactory = new InMemoryKsiSignatureComponentFactory();
          KSISignatureFactory signatureFactory = new InMemoryKsiSignatureFactory(policy, signatureComponentFactory);
          return new ExtenderImpl(extendingService, publicationsHandler, signatureFactory, signatureComponentFactory);
      }
docs: Builds the {@link Extender} instance. Checks that the extender and the publications file handler are set.
  If not configured, {@link NullPointerException} is thrown.
  
  @return Instance of {@link Extender} class.
  @throws KSIException will be thrown when errors occur on {@link Extender} class initialization.