PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features) → tried but not able to come up with a reproduction
• Docs have been added / updated (for bug fixes / features) → not needed IMO

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

When creating a control with an async validator instantly resolved with for example of(null), the extended class (Angular native control) will call the method internally during the class instantiation through the super causing a throw of error since the code except that the setErrors has a valid not nil errorsSubject.
The constructor is creating just after this the subject.

Issue Number: #91

What is the new behavior?

The control's method setErrors will no longer throw an error when errorsSubject is not yet created.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

Fixes #91