check endpoint first

projects/nshmp-ws/src/app/swagger/app.component.ts

+import {HttpClient} from '@angular/common/http';
 import {
   Component,
   ElementRef,
@@ -11,8 +12,10 @@ import {
   NshmpLibNgAlertComponent,
   NshmpLibNgSpinnerComponent,
   NshmpLibNgTemplateComponent,
+  NshmpService,
   SpinnerService,
 } from '@ghsc/nshmp-lib-ng/nshmp';
+import {catchError} from 'rxjs';
 import {Spec, SwaggerUIBundle} from 'swagger-ui-dist';
 
 import {NavigationService} from '../navigation.service';
@@ -37,6 +40,8 @@ export class AppComponent implements OnInit {
   constructor(
     public navigationService: NavigationService,
     private spinnerService: SpinnerService,
+    private httpClient: HttpClient,
+    private nshmpService: NshmpService,
   ) {}
 
   ngOnInit(): void {
@@ -45,23 +50,36 @@ export class AppComponent implements OnInit {
     const contextPath = location.pathname;
     const url = `${location.href}/swagger`;
 
-    SwaggerUIBundle({
-      deepLinking: false,
-      defaultModelsExpandDepth: 0,
-      domNode: this.swagger().nativeElement,
-      layout: 'BaseLayout',
-      plugins: [
-        // eslint-disable-next-line @typescript-eslint/no-unsafe-member-access
-        SwaggerUIBundle?.plugins?.DownloadUrl,
-        this.updateContextPath(contextPath),
-      ],
-      // eslint-disable-next-line @typescript-eslint/no-unsafe-member-access
-      presets: [SwaggerUIBundle?.presets?.apis],
-      tagsSorter: 'alpha',
-      tryItOutEnabled: true,
-      url,
-      validatorUrl: null,
-    });
+    // Check swagger endpoint first
+    this.httpClient
+      .get(url)
+      .pipe(
+        catchError((error: Error) => {
+          this.spinnerRef.close();
+          return this.nshmpService.throwError$(
+            new Error(`Failed to get swagger endpoint. ${error.message}`),
+          );
+        }),
+      )
+      .subscribe(() => {
+        SwaggerUIBundle({
+          deepLinking: false,
+          defaultModelsExpandDepth: 0,
+          domNode: this.swagger().nativeElement,
+          layout: 'BaseLayout',
+          plugins: [
+            // eslint-disable-next-line @typescript-eslint/no-unsafe-member-access
+            SwaggerUIBundle?.plugins?.DownloadUrl,
+            this.updateContextPath(contextPath),
+          ],
+          // eslint-disable-next-line @typescript-eslint/no-unsafe-member-access
+          presets: [SwaggerUIBundle?.presets?.apis],
+          tagsSorter: 'alpha',
+          tryItOutEnabled: true,
+          url,
+          validatorUrl: null,
+        });
+      });
   }
 
   updateContextPath(contextPath: string) {