How to conditionally add [matMenuTriggerFor] in mat-menu

Few #menu items has no sub child. But shows the arrow which indicates there is a submenu. Which is wrong. How to add [matMenuTriggerFor]="subMenu" only when there are items in subMenu.

<mat-menu #menu="matMenu">
<ng-container *ngFor="let item of menuItem.items; let i=index">
<button class="mat-menu-item" mat-menu-item
[routerLink]="item.routerLink" routerLinkActive="active" [matMenuTriggerFor]="subMenu"> {{item.name}}
</button>
<mat-menu #subMenu="matMenu"> <button class="mat-sub-menu-item"
*ngFor="let subitem of menuItem.items.subitems" mat-menu-item> {{ subitem.name }} </button>
</mat-menu>
</ng-container>
</mat-menu>

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