client/src/App.js

@@ -13,7 +13,11 @@ class App extends Component {
                 <Switch>
                     {
                         mainMenuItems.map(mainMenuItem =>
-                            <Route exact path={mainMenuItem.url} component={mainMenuItem.component} />)
+                            <Route key={mainMenuItem.id}
+                                   exact
+                                   path={mainMenuItem.url}
+                                   component={mainMenuItem.component}
+                            />)
                     }
                     <Route component={NotFound} />
                 </Switch>

client/src/components/MainMenu/index.js

@@ -8,7 +8,13 @@ const MainMenu = () =>
     (<Menu>
         {
             mainMenuItems.map(mainMenuItem =>
-                <Menu.Item as={NavLink} to={mainMenuItem.url} exact>{mainMenuItem.title}</Menu.Item>)
+                <Menu.Item key={mainMenuItem.id}
+                           as={NavLink}
+                           to={mainMenuItem.url}
+                           exact
+                >
+                    {mainMenuItem.title}
+                </Menu.Item>)
         }
     </Menu>);