Disable "new session" side menu entry when no calculator is open

src/app/app.component.html

@@ -86,7 +86,11 @@
           <mat-icon>add</mat-icon>
           {{ uitextSidenavNewCalc }}
         </a>
-        <a id="side-nav-empty-session" (click)="sidenav.close(); emptySession()">
+        <a *ngIf="enableEmptySessionMenu" id="side-nav-empty-session" (click)="sidenav.close(); emptySession()">
+            <mat-icon>insert_drive_file</mat-icon>
+            {{ uitextSidenavEmptySession }}
+        </a>
+        <a *ngIf="! enableEmptySessionMenu" id="side-nav-empty-session" class="disabled-link">
             <mat-icon>insert_drive_file</mat-icon>
             {{ uitextSidenavEmptySession }}
         </a>

src/app/app.component.ts

@@ -305,6 +305,10 @@ export class AppComponent implements OnInit, OnDestroy, Observer {
     return this._calculators.length > 0;
   }
 
+  public get enableEmptySessionMenu(): boolean {
+    return this._calculators.length > 0;
+  }
+
   // interface Observer
 
   update(sender: any, data: any): void {