fix item-page 404 redirect

src/app/+item-page/full/full-item-page.component.ts

 import {filter, map} from 'rxjs/operators';
 import { ChangeDetectionStrategy, Component, OnInit } from '@angular/core';
-import { ActivatedRoute } from '@angular/router';
+import { ActivatedRoute, Router } from '@angular/router';
 
 import { Observable ,  BehaviorSubject } from 'rxjs';
 
@@ -35,8 +35,8 @@ export class FullItemPageComponent extends ItemPageComponent implements OnInit {
 
   metadata$: Observable<MetadataMap>;
 
-  constructor(route: ActivatedRoute, items: ItemDataService, metadataService: MetadataService) {
-    super(route, items, metadataService);
+  constructor(route: ActivatedRoute, router: Router, items: ItemDataService, metadataService: MetadataService) {
+    super(route, router, items, metadataService);
   }
 
   /*** AoT inheritance fix, will hopefully be resolved in the near future **/

src/app/+item-page/item-page.resolver.ts

 import { Injectable } from '@angular/core';
 import { ActivatedRouteSnapshot, Resolve, Router, RouterStateSnapshot } from '@angular/router';
-import { EMPTY, Observable } from 'rxjs';
+import { Observable } from 'rxjs';
 import { RemoteData } from '../core/data/remote-data';
-import { getSucceededRemoteData } from '../core/shared/operators';
 import { ItemDataService } from '../core/data/item-data.service';
 import { Item } from '../core/shared/item.model';
 import { hasValue } from '../shared/empty.util';
@@ -13,7 +12,7 @@ import { find, map } from 'rxjs/operators';
  */
 @Injectable()
 export class ItemPageResolver implements Resolve<RemoteData<Item>> {
-  constructor(private itemService: ItemDataService, private router: Router) {
+  constructor(private itemService: ItemDataService) {
   }
 
   /**
@@ -26,13 +25,6 @@ export class ItemPageResolver implements Resolve<RemoteData<Item>> {
     return this.itemService.findById(route.params.id)
       .pipe(
         find((RD) => hasValue(RD.error) || RD.hasSucceeded),
-        map((RD) => {
-          if (hasValue(RD.error)) {
-            this.router.navigateByUrl('/404', { skipLocationChange: true });
-            return null;
-          }
-          return RD;
-        })
       );
   }
 }

src/app/+item-page/simple/item-page.component.ts

-import { filter, map, mergeMap } from 'rxjs/operators';
+
+import { mergeMap, filter, map, take } from 'rxjs/operators';
 import { ChangeDetectionStrategy, Component, OnInit } from '@angular/core';
-import { ActivatedRoute } from '@angular/router';
+import { ActivatedRoute, Router } from '@angular/router';
 
 import { Observable } from 'rxjs';
 import { ItemDataService } from '../../core/data/item-data.service';
@@ -51,12 +52,18 @@ export class ItemPageComponent implements OnInit {
 
   constructor(
     private route: ActivatedRoute,
+    private router: Router,
     private items: ItemDataService,
     private metadataService: MetadataService,
   ) { }
 
   ngOnInit(): void {
     this.itemRD$ = this.route.data.pipe(map((data) => data.item));
+    this.itemRD$.pipe(take(1)).subscribe((itemRD: RemoteData<Item>) => {
+      if (itemRD.hasFailed && itemRD.error.statusCode === 404) {
+        this.router.navigateByUrl('/404', { skipLocationChange: true });
+      }
+    });
     this.metadataService.processRemoteData(this.itemRD$);
     this.thumbnail$ = this.itemRD$.pipe(
       map((rd: RemoteData<Item>) => rd.payload),