Angular Dropdown list validation

I am beginner to Angular

I have a task to add validation if Dropdown list is not selected I have to hide button

here is that code

button function

<input type="file" class="custom-file-input" multiple (change)="onDealPriceFileChangeHandler($event)" accept=".xlsx"   />

Dropdown list code

<div class="form-group">
   <label class="label" for="nights">Number of Nights</label>

   <nb-select fullWidth  placeholder="Select Number of Nights" >

 <nb-option *ngFor="let night of nights" [value]="night.value"
  (selectionChange)="nightHanlder(night.value)">{{night.name}}
  </nb-option>

 </nb-select>

 </div>

Night Array

nights = [
      { value: 1, name: '1 Night' },
      { value: 2, name: '2 Nights' },
      { value: 3, name: '3 Nights' },
]

night change function

nightHanlder(code){
   
this.selectNight=code;
}

I need to add an alert if dropdown is not selected