[Validator] Fixed the wrong isAbstract() call in PR #8592

| Q             | A
| ------------- | ---
| Bug fix?      | [yes]
| New feature?  | [no]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes]
| Fixed tickets | [#8589]
| License       | MIT
| Doc PR        | [n/a]