Angular Reactive Forms - Validation Errors - Cannot read property of undefined or null

I'm having a heck of a time understanding something that should be easy. Form validation. I'm using reactive forms for the first time. I'm using Form Builder. I've got a nested set of controls. In general, I'm assuming you create a set of controls with validation, then check for those in the template. I'm able to submit the form but keep getting errors when trying to create validation messages.

------ TEMPLATE -------

<div class="container">
  <div class="row">
    <div class="col-sm">

      <!-- Begin card -->
      <div class="card border">
        <div class="card-header bg-light text-primary">
          <h3>My Account</h3>
        </div>
        <div class="card-body border border-light">
          <!-- CHANGE PASSWORD BUTTON -->
          <div class="row">
            <div class="col-sm">
              <button class="btn btn-success" (click)="goToChangePassword()">Change Password</button>
            </div>
          </div>
          <!-- Begin Form -->
          <form *ngIf="user; else loading" [formGroup]="accountForm" (ngSubmit)="updateUser()">
            <hr>
            <!-- Phone -->
            <fieldset formGroupName="phone">
              <div class="form-row">
                <div class="form-group col-sm">
                  <label>Home Phone</label>
                  <input type="number" class="form-control" formControlName="home">
                  <div *ngIf="isSubmitted && fControls.home.errors" class="invalid-feedback">
                    <div *ngIf="fControls.home.errors.minlength">10 characters min</div>
                  </div>
                </div>
                <div class="form-group col-sm">
                  <label>Mobile Phone</label>
                  <input type="number" class="form-control" formControlName="mobile">
                  <div *ngIf="isSubmitted && fControls.mobile.errors" class="invalid-feedback">
                    <div *ngIf="fControls.mobile.errors.minlength">10 characters min</div>
                  </div>
                </div>
                <div class="form-group col-sm">
                  <label>Extension</label>
                  <input type="number" class="form-control" formControlName="extension">
                  <div *ngIf="isSubmitted && fControls.extension.errors" class="invalid-feedback">
                    <div *ngIf="fControls.extension.errors.maxlength">5 chartacters max</div>
                  </div>
                </div>
              </div>
            </fieldset>

            <!-- BIRTHDAY -->
            <fieldset formGroupName="birthday">
              <div class="form-row">
                <div class="form-group col-sm-3">
                  <label>Birthday Month</label>
                  <input type="number" class="form-control" formControlName="month">
                </div>
                <div class="form-group col-sm-3">
                  <label>Birthday Day</label>
                  <input type="number" class="form-control" formControlName="day">
                </div>
              </div>
            </fieldset>
            <hr />

            <!-- BUTTONS -->
            <div class=" form-row">
              <button type="submit" class="btn btn-success mr-3" [disabled]="!accountForm.valid">Update</button>
              <button class="btn btn-danger" (click)="cancel()">Cancel</button>
            </div>


            <!-- End form -->
          </form>
        </div>
      </div>
    </div>
  </div>
</div>

<!-- LOADING TEMPLATE -->
<ng-template #loading>
  Loading User...
</ng-template>



------ TS --------

import { Component, OnInit } from '@angular/core';
import { UserService } from 'src/app/services/user.service';
import { FormBuilder, FormGroup, Validators } from '@angular/forms';
import { Router } from '@angular/router';
import { AppToastService } from 'src/app/services/toastr.service';

@Component({
  selector: 'app-account',
  templateUrl: './account.component.html',
  styleUrls: ['./account.component.css']
})
export class AccountComponent implements OnInit {
  user;
  userId;
  accountForm: FormGroup;
  isSubmitted = false;

  constructor(
    private _userService: UserService,
    private _router: Router,
    private _toast: AppToastService,
    private fb: FormBuilder
  ) {}

  ngOnInit() {
    // Initiate the form
    this.accountForm = this.fb.group({
      phone: this.fb.group({
        home: [' ', Validators.minLength(10)],
        mobile: [' ', Validators.minLength(10)],
        extension: [' ', Validators.maxLength(5)]
      }),
      birthday: this.fb.group({
        month: '',
        day: ''
      })
    });

    // Get userId from localstorage, then fetch the details
    this.userId = this._userService.getUserIdFromLocalStorage();

    // Get user from userId
    this._userService.getUserAccountInfo(this.userId).subscribe(
      response => {
        this.user = response['user'];
        this.accountForm.patchValue(response['user']);
      },
      error => console.log(error['message'])
    );
  }

  // Access form controls
  get fControls() {
    return this.accountForm.controls;
  }

  updateUser() {
    // Check for form validity
    this.isSubmitted = true;
    if (this.accountForm.invalid) {
      return;
    }

    let updatedUser = this.accountForm.value;
    updatedUser.id = this.userId;

    console.log(updatedUser);

  }