code: function findInstance(handle, shouldWait) {
      var instance = $mdComponentRegistry.get(handle);
  
      if (!instance && !shouldWait) {
        // Report missing instance
        $log.error($mdUtil.supplant(errorMsg, [handle || ""]));
  
        // The component has not registered itself... most like NOT yet created
        // return null to indicate that the Sidenav is not in the DOM
        return undefined;
      }
      return instance;
    }
docs: Synchronously lookup the controller instance for the specified sidNav instance which has been
  registered with the markup `md-component-id`