no static property in session service, add spec

frontend/src/app/services/session.service.spec.ts

@@ -0,0 +1,16 @@
+import { TestBed } from '@angular/core/testing';
+import { SessionService } from './session.service';
+
+describe('SessionService', () => {
+    let service: SessionService;
+
+    beforeEach(() => {
+        TestBed.configureTestingModule({});
+        service = TestBed.inject(SessionService);
+    });
+
+    it('should be created', () => {
+        expect(service).toBeTruthy();
+    });
+
+});

frontend/src/app/services/session.service.ts

@@ -5,10 +5,10 @@ import { Subject } from 'rxjs';
     providedIn: 'root'
 })
 export class SessionService {
-    private static sessionExpiredSubject = new Subject<void>();
-    public expired = SessionService.sessionExpiredSubject.asObservable();
+    private sessionExpiredSubject = new Subject<void>();
+    public expired = this.sessionExpiredSubject.asObservable();
 
-    public static markExpired() {
-        SessionService.sessionExpiredSubject.next();
+    public markExpired() {
+        this.sessionExpiredSubject.next();
     }
 }