Добавил редирект на 404-ю стр.

src/public/js/react/site/index.js

@@ -86,6 +86,14 @@ const Home = () => (
 )
 
 
+const My404Component = () => (
+    <div>
+        <h1>Welcome to 404!</h1>
+        <h1>Page not found</h1>
+    </div>
+)
+
+
 const SiteForm = () => (
 <div className="site">
     <h2>Технические данные</h2>
@@ -191,6 +199,9 @@ const Main = () => (
                     <PrivateRoute path='/sites' component={Sites}/>
                     <PrivateRoute path='/site-form' component={SiteForm}/>
                     <Route path="/login" component={Login} />
+                    
+                    <Route path='/404' component={My404Component} />
+                    <Redirect from='*' to='/404' />
                 </Switch>
             </div>
         </section>