Warning on submit "key() expects parameter 1 to be array, string given"

Submitting a simple form with no captcha (similar to #228)

Whoops\Exception\ErrorException thrown with message "key() expects parameter 1 to be array, string given"

```
if (is_numeric($action)) {
$action = \key($data); // <--- This line
$data = $data[$action];
}
```

```
Stacktrace:
#50 Whoops\Exception\ErrorException in /var/www/myproject/user/plugins/form/classes/Form.php:885
#49 key in /var/www/myproject/user/plugins/form/classes/Form.php:885
#48 Grav\Plugin\Form\Form:post in /var/www/myproject/user/plugins/form/form.php:258
#47 Grav\Plugin\FormPlugin:onPageInitialized in /var/www/myproject/vendor/symfony/event-dispatcher/EventDispatcher.php:212
#46 Symfony\Component\EventDispatcher\EventDispatcher:doDispatch in /var/www/myproject/vendor/symfony/event-dispatcher/EventDispatcher.php:44
#45 Symfony\Component\EventDispatcher\EventDispatcher:dispatch in /var/www/myproject/vendor/rockettheme/toolbox/Event/src/EventDispatcher.php:23
#44 RocketTheme\Toolbox\Event\EventDispatcher:dispatch in /var/www/myproject/system/src/Grav/Common/Grav.php:403
#43 Grav\Common\Grav:fireEvent in /var/www/myproject/system/src/Grav/Common/Processors/PagesProcessor.php:32
#42 Grav\Common\Processors\PagesProcessor:process in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:45
#41 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:57
#40 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Common/Processors/TwigProcessor.php:27
#39 Grav\Common\Processors\TwigProcessor:process in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:45
#38 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:57
#37 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Common/Processors/AssetsProcessor.php:28
#36 Grav\Common\Processors\AssetsProcessor:process in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:45
#35 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:57
#34 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Common/Processors/SchedulerProcessor.php:29
#33 Grav\Common\Processors\SchedulerProcessor:process in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:45
#32 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:57
#31 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Common/Processors/BackupsProcessor.php:28
#30 Grav\Common\Processors\BackupsProcessor:process in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:45
#29 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:57
#28 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Common/Processors/TasksProcessor.php:59
#27 Grav\Common\Processors\TasksProcessor:process in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:45
#26 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:57
#25 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Common/Processors/RequestProcessor.php:52
#24 Grav\Common\Processors\RequestProcessor:process in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:45
#23 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:57
#22 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Common/Processors/ThemesProcessor.php:27
#21 Grav\Common\Processors\ThemesProcessor:process in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:45
#20 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:57
#19 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Common/Processors/PluginsProcessor.php:30
#18 Grav\Common\Processors\PluginsProcessor:process in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:45
#17 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:57
#16 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Common/Processors/InitializeProcessor.php:95
#15 Grav\Common\Processors\InitializeProcessor:process in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:45
#14 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:57
#13 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Common/Processors/DebuggerProcessor.php:27
#12 Grav\Common\Processors\DebuggerProcessor:process in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:45
#11 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:57
#10 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Common/Processors/ErrorsProcessor.php:27
#9 Grav\Common\Processors\ErrorsProcessor:process in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:45
#8 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:57
#7 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Common/Processors/LoggerProcessor.php:48
#6 Grav\Common\Processors\LoggerProcessor:process in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:45
#5 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:57
#4 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Common/Processors/ConfigurationProcessor.php:28
#3 Grav\Common\Processors\ConfigurationProcessor:process in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:45
#2 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Framework/RequestHandler/Traits/RequestHandlerTrait.php:57
#1 Grav\Framework\RequestHandler\RequestHandler:handle in /var/www/myproject/system/src/Grav/Common/Grav.php:272
#0 Grav\Common\Grav:process in /var/www/myproject/index.php:54
```

Start in user/plugins/form/classes/Form.php at line 885, following the post flow from user/plugins/form/form.php. Reproduce a simple form submission without captcha, inspect the data passed to key(), and verify that submission completes without the reported warning.

Written by the indexing model from the issue text.