Angular - BehaviorSubject is always undefined in service

I have some problem with this auth service.

I created one for authentication. I want to set my user if i can log in. On the call, when i want to call this.user.next(user), i get an error. The error says, that this.user is undefined. But why? I have declared it.

How can i fix this?

Here is my code:

import { HttpClient, HttpErrorResponse, HttpParams } from "@angular/common/http";
import { Injectable } from "@angular/core";
import { BehaviorSubject, catchError, tap, throwError } from "rxjs";
import { User } from "./user.model";

export interface AuthResponseData {
...
}
@Injectable({
providedIn: 'root'
})
export class AuthService {
private apiKey = '...'
user = new BehaviorSubject<User>(null)

constructor(
private http: HttpClient
) {}

signup(email: string, password: string) {
...
).pipe(
catchError(this.handleError),
tap(this.handleAuthentication)
)
}

login(email: string, password: string) {
...
).pipe(
catchError(this.handleError),
tap(this.handleAuthentication)
)
}

private handleAuthentication(response: AuthResponseData) {
const { email, localId, idToken, expiresIn } = response
const expires = new Date(new Date().getTime() + +expiresIn * 1000)
const user = new User(email, localId, idToken, expires)
this.user.next(user)
}

private handleError(errorResponse: HttpErrorResponse) {
...
}
}

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