code: public function getField($name)
      {
          if ($this->fields === null) {
              $this->getFields();
          }
          Utils::invariant(isset($this->fields[$name]), 'Field "%s" is not defined for type "%s"', $name, $this->name);
  
          return $this->fields[$name];
      }
docs: @param string $name
  
  @return FieldDefinition
  
  @throws Exception