Steps to reproduce the issue

Install on Joomla 4 fakecaptcha.zip plugin. It always fails the captcha test. Enable the plugin and set as default in global configuration. Open frontend registration, fill the data and submit the form.

Expected result

User is redirect back to form with error message, that captcha test failed.

Actual result

Error page is displayed - Call to undefined method Joomla\CMS\Captcha\Captcha::getError()

System information (as much as possible)

Joomla 4 alpha 4

Additional comments

There seems to be unported code in Joomla\CMS\Captcha\Captcha - its trying to check getError() which does not exists in Joomla 4 anymore.

Following code in recaptcha plugin (it throws exception on validation failed) will not raise this error, however will show error page to user instead of showing him just error message and giving chance to try the captcha again.

Joomla 3 uses $this->_subject to set those error messages, this is missing in Joomla 4, but can easily be bypassed by directly using Application::enqueueMessage.