code: public void assertNotNull(Object propertyValue, String propertyName) {
          if (propertyValue == null) {
              problemReporter.report(new Problem(this, String.format("%s is a required property.", propertyName)));
          }
      }
docs: Asserts the value is not null, reporting to {@link ProblemReporter} with this context if it is.
  
  @param propertyValue Value to assert on.
  @param propertyName  Name of property.