Button is aligned right in the toolbar but the icon is not shown

Ionic Framework Ionic Angular
1

Button is aligned right in the toolbar but the icon is not shown.

Only white box appear.

.html

<ion-header>
  <ion-toolbar>
    <ion-buttons slot="start">
      <ion-menu-button></ion-menu-button>
    </ion-buttons>

    <ion-title style="text-align: center;">RE-MED+</ion-title>

    <ion-buttons slot="primary">
      <ion-button>
        <ion-icon slot="icon-only" name="exit"></ion-icon>
      </ion-button>
    </ion-buttons>

  </ion-toolbar>
</ion-header>

It looks like this.

enter image description here

I want Like this

enter image description here