Can’t construct a query for the property, since the query selector wasn’t defined

It’s because of a circular dependency between SidenavComponent and SidenavNavComponent. It can be resolved using forwardRef. Don’t forget to import it as well:

import { forwardRef } from '@angular/core';

@ViewChild(forwardRef(() => SidenavNavComponent))
private navComponent: SidenavNavComponent;

Plunker example