Make sure parent default formatter gets assgined.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | .
| License       | MIT
| Doc PR        | .

The parent constructor will create a new formatter if $formatter is null.

This avoids that the formatter becomes 2 different instances in $this and $this->stderr, if null was passed.