[Form] Fixed memory leak in FormValidator
[Form] Added method Form::getClickedButton() to remove memory leak in FormValidator
bug #9211 [Form] Fixed memory leak in FormValidator (bschussek)

This PR was merged into the 2.3 branch.

Discussion
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[Form] Fixed memory leak in FormValidator

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #9187
| License       | MIT
| Doc PR        | -

Commits
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1bb7e4d [Form] Added method Form::getClickedButton() to remove memory leak in FormValidator
5329ab5 [Form] Fixed memory leak in FormValidator