Angular 12 - TypeError: Cannot read properties of null (reading 'writeValue')

Question

Angular 12 - TypeError: Cannot read properties of null (reading 'writeValue')

I'm creating a generic text input component, everything works well while serving the project, but in the built project I get this error:

TypeError: Cannot read properties of null (reading 'writeValue')

HTML:

<div class="p-field">
    <label>{{label}} <span class="p-error">{{checkRequired(ngControl.control) ? '*' : ''}}</span></label>
    <input class="full-width" [type]="type" pInputText [formControl]="ngControl.control">

    <small *ngIf="ngControl.control.touched && ngControl.control.errors?.required" class="p-error">{{label}} is required</small>


    <small *ngIf="ngControl.control.errors?.minlength" class="p-error">
        {{label}} must be at least {{ngControl.control.errors.minlength['requiredLength']}}
    </small>

    <small *ngIf="ngControl.control.errors?.maxlength" class="p-error">
        {{label}} must be at most {{ngControl.control.errors.maxlength['requiredLength']}}
    </small>
    <small *ngIf="ngControl.control.errors?.email" class="p-error">
        This email is not valid
    </small>

    <small *ngIf="ngControl.control.errors?.isMatching" class="p-error">Passwords do not match</small>
</div>

component.ts

import { Component, Input, OnInit, Self } from '@angular/core';
import {AbstractControl, ControlValueAccessor, NgControl} from '@angular/forms';

@Component({
  selector: 'app-text-input',
  templateUrl: './text-input.component.html',
  styleUrls: ['./text-input.component.css']
})
export class TextInputComponent implements ControlValueAccessor {
  @Input() label: string;
  @Input() type = 'text';

  constructor(@Self() public ngControl: NgControl) {
    this.ngControl.valueAccessor = this;
  }

  checkRequired(control) {
    if (control.validator) {
      const validator = control.validator({} as AbstractControl);
      if (validator && validator.required) {
        return true;
      }
    }
  }

  writeValue(obj: any): void {
  }

  registerOnChange(fn: any): void {
  }

  registerOnTouched(fn: any): void {
  }

}

Angular info:

Angular CLI: 12.2.6 Node: 14.17.3 Package Manager: npm 6.14.13 OS: win32 x64 Angular: 12.2.6

Answer 1

In my case, the specific error was appearing because I had not added the specific component in the module imports.

Answer 2

The error occurs when provider setup is missing

  providers: [
    {
      provide: NG_VALUE_ACCESSOR,
      multi:true,
      useExisting: CUSTOMER_INPUT_CLASS_HERE
    }
  ]  

In your case

@Component({
  selector: 'app-text-input',
  templateUrl: './text-input.component.html',
  styleUrls: ['./text-input.component.css'],
  providers: [
    {
      provide: NG_VALUE_ACCESSOR,
      multi:true,
      useExisting: TextInputComponent
    }
  ]
})
export class TextInputComponent implements ControlValueAccessor {
...
}