fix(combo): fix potential combo memory leak, #4928

Author: ViktorSlavov

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

@@ -1307,6 +1307,7 @@ export class IgxComboComponent extends DisplayDensityBase implements IgxComboBas
      * @hidden @internal
      */
     public ngOnDestroy() {
+        this.destroy$.next();
         this.destroy$.complete();
         this.comboAPI.clear();
         this.selection.clear(this.id);