fix(control): avoid throw of error with async validator

fixes #91

projects/ngneat/reactive-forms/src/lib/formControl.ts

@@ -1,4 +1,4 @@
-import { AbstractControl, FormControl as NgFormControl } from '@angular/forms';
+import { FormControl as NgFormControl } from '@angular/forms';
 import { isObservable, Observable, Subject, Subscription } from 'rxjs';
 import { distinctUntilChanged } from 'rxjs/operators';
 import {
@@ -159,7 +159,17 @@ export class FormControl<T = any, E extends object = any> extends NgFormControl
  }
 
  setErrors(errors: Partial<E> | null, opts: EmitEvent = {}) {
- this.errorsSubject.next(errors);
+ /**
+ * @description
+ * Use an elvis operator to avoid a throw when the control is used with an async validator
+ * Which will be instantly resolved (like with `of(null)`)
+ * In such case, Angular will call this method instantly before even instancing the properties causing the throw
+ * Can be easily reproduced with a step-by-step debug once compiled when checking the stack trace of the constructor
+ *
+ * Issue: https://github.com/ngneat/reactive-forms/issues/91
+ * Reproduction: https://codesandbox.io/embed/github/C0ZEN/ngneat-reactive-forms-error-issue-cs/tree/main/?autoresize=1&expanddevtools=1&fontsize=14&hidenavigation=1&theme=dark
+ */
+ this.errorsSubject?.next(errors);
  return super.setErrors(errors, opts);
  }