chore(combo): apply suggestions

Author: ddaribo

projects/igniteui-angular/combo/src/combo/combo-dropdown.component.ts

@@ -59,7 +59,7 @@ export class IgxComboDropDownComponent extends IgxDropDownComponent implements I
     /**
      * @hidden @internal
      */
-    public override get focusedItem(): IgxDropDownItemBaseDirective {
+    public override get focusedItem(): IgxDropDownItemBaseDirective | null {
         return super.focusedItem;
     }
 
@@ -75,17 +75,15 @@ export class IgxComboDropDownComponent extends IgxDropDownComponent implements I
     }
 
     /** @hidden @internal */
-    public override set focusedItem(item: IgxDropDownItemBaseDirective) {
+    public override set focusedItem(item: IgxDropDownItemBaseDirective | null) {
         if (!item) {
             this._activeDescendantId = null;
         } else if (item.id !== undefined) {
             this._activeDescendantId = item.id;
         } else {
             // Virtual { value, index } object passed by navigateItem() under virtual scrolling.
             const resolved = this.children?.find(e => e.index === item.index);
-            if (resolved) {
-                this._activeDescendantId = resolved.id;
-            }
+            this._activeDescendantId = resolved?.id ?? null;
         }
         super.focusedItem = item;
     }
@@ -177,7 +175,7 @@ export class IgxComboDropDownComponent extends IgxDropDownComponent implements I
             return;
         }
         this.comboAPI.set_selected_item(item.itemID);
-        this._activeDescendantId = item.id;
+        this._activeDescendantId = item.id ?? null;
         this._focusedItem = item;
         this.combo.setActiveDescendant();
     }

projects/igniteui-angular/combo/src/combo/combo.component.spec.ts

@@ -3709,13 +3709,14 @@ describe('igxCombo', () => {
 
         describe('Zoneless', () => {
             beforeEach(async () => {
+                TestBed.resetTestingModule();
                 await TestBed.configureTestingModule({
                     imports: [
                         NoopAnimationsModule,
-                        IgxComboComponent,
+                        IgxComboSampleComponent,
                     ],
                     providers: [
-                        provideZonelessChangeDetection()
+                        provideZonelessChangeDetection(),
                     ]
                 }).compileComponents();
 
@@ -3724,21 +3725,23 @@ describe('igxCombo', () => {
                 combo = fixture.componentInstance.combo;
             });
 
-            it('should not reproduce NG0100 when virtualized combo items update on scroll - issue #17310', async () => {
+            it('should not reproduce NG0100 when virtualized combo items update on scroll - issue #17310', fakeAsync(() => {
                 combo.open();
-                await fixture.whenStable();
+                tick();
+                fixture.detectChanges();
 
                 const scrollEl = combo.virtualScrollContainer.getScroll();
                 expect(scrollEl).toBeTruthy();
 
+                scrollEl.scrollTop = 300;
+                scrollEl.dispatchEvent(new Event('scroll'));
+                tick(100);
+
                 expect(() => {
-                    scrollEl.scrollTop = 300;
-                    scrollEl.dispatchEvent(new Event('scroll'));
 
-                    fixture.whenStable();
                     fixture.detectChanges();
                 }).not.toThrowError(/NG0100|ExpressionChangedAfterItHasBeenCheckedError/);
-            });
+            }));
 
             it('should not reproduce NG0100 related to active descendant on scroll - issue #17310', fakeAsync(() => {
                 combo.toggle();

projects/igniteui-angular/drop-down/src/drop-down/drop-down.base.ts

@@ -173,7 +173,7 @@ export abstract class IgxDropDownBaseDirective implements IDropDownList, OnInit
      * the IgxDropDownNavigationDirective host element.
      */
     public get activeDescendant (): string | null {
-        return this.focusedItem ? this.focusedItem.id : null;
+        return this.focusedItem?.id ?? null;
     }
 
     /**