code: public function setModule($id, $module)
      {
          if ($module === null) {
              unset($this->_modules[$id]);
          } else {
              $this->_modules[$id] = $module;
          }
      }
docs: Adds a sub-module to this module.
  @param string $id module ID.
  @param Module|array|null $module the sub-module to be added to this module. This can
  be one of the following:
  
  - a [[Module]] object
  - a configuration array: when [[getModule()]] is called initially, the array
  will be used to instantiate the sub-module
  - `null`: the named sub-module will be removed from this module