Argument of type 'Element | null' is not assignable to parameter of type 'Element'. Type...

I'm trying to create an Angular stepper form with bs-stepper, I have installed npm and add the CDN inside index.html.

When I implement the code in my componenet.ts, I get an error:

Argument of type 'Element | null' is not assignable to parameter of type 'Element'.
Type 'null' is not assignable to type 'Element'.

51 this.stepper = new Stepper(document.querySelector('#stepper1'), {
​

This is my component.ts:

import Stepper from 'bs-stepper'
export class RegistreComponent implements OnInit {
private stepper!: Stepper;
next() {
this.stepper.next();
}
onSubmit() {
return false;
}

ngOnInit(): void {
this.stepper = new Stepper(document.querySelector('#stepper1'), {
linear: false,
animation: true
})
}
}

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