fixed an issue where the reducers wouldn't be combined correctly when using AoT compilation

src/app/app.module.ts

@@ -11,7 +11,7 @@ import { StoreModule } from "@ngrx/store";
 import { RouterStoreModule } from "@ngrx/router-store";
 import { StoreDevtoolsModule } from "@ngrx/store-devtools";
 
-import { reducers } from './app.reducers';
+import { rootReducer } from './app.reducers';
 import { effects } from './app.effects';
 
 @NgModule({
@@ -30,7 +30,7 @@ import { effects } from './app.effects';
      * meta-reducer. This returns all providers for an @ngrx/store
      * based application.
      */
-    StoreModule.provideStore(reducers),
+    StoreModule.provideStore(rootReducer),
 
     /**
      * @ngrx/router-store keeps router state up-to-date in the store and uses

src/app/app.reducers.ts

@@ -1,7 +1,20 @@
-import { headerReducer } from './header/header.reducer';
-import { hostWindowReducer } from "./shared/host-window.reducer";
+import { combineReducers } from "@ngrx/store";
+import { routerReducer, RouterState } from "@ngrx/router-store";
+import { headerReducer, HeaderState } from './header/header.reducer';
+import { hostWindowReducer, HostWindowState } from "./shared/host-window.reducer";
+
+export interface AppState {
+  router: RouterState;
+  hostWindow: HostWindowState;
+  header: HeaderState;
+}
 
 export const reducers = {
-  headerReducer,
-  hostWindowReducer
+  router: routerReducer,
+  hostWindow: hostWindowReducer,
+  header: headerReducer
+};
+
+export function rootReducer(state: any, action: any) {
+  return combineReducers(reducers)(state, action);
 }

src/app/header/header.component.ts

@@ -19,9 +19,7 @@ export class HeaderComponent implements OnInit {
   }
 
   ngOnInit(): void {
-    this.isNavBarCollapsed = this.store.select('headerReducer')
-      //ensure that state is not null, can happen when using AoT compilation
-      .filter((state: HeaderState) => state !== null && state !== undefined)
+    this.isNavBarCollapsed = this.store.select('header')
       //unwrap navCollapsed
       .map(({ navCollapsed }: HeaderState) => navCollapsed);
   }