I want to add a "Delete All" action button to the Userforms Submissions Gridfield. I have created the code for the button in another file.

On the page that extends UserDefinedForm I am trying to get the existing inherited Submissions gridfield and add another component (the button) to it:

class CompetitionFormPage extends UserDefinedForm  {

	static $db = array(
		'Details' => 'HTMLText'
	);

	public function getCMSFields() {
            $fields = parent::getCMSFields();

            $DetailsField = new HtmlEditorField('Details', 'Details');

            $fields->addFieldsToTab('Root.Main', array(
        	new HtmlEditorField('Details', 'Details')
             ));

        
             //get exisitng Submissions gridfield
            $gridField = $fields->fieldByName('Submissions');
            $gridField->getConfig()->addComponent(new GridFieldDeleteAllButton());

             return $fields;
	}


}

class CompetitionFormPage_Controller extends UserDefinedForm_Controller {

}

This results in a Fatal error: Call to a member function getConfig() on a non-object

For reference here is the GridFieldDeleteAllButton

class GridFieldDeleteAllButton implements GridField_HTMLProvider, GridField_ActionProvider {
	protected $targetFragment;

	public function __construct($targetFragment = "before") {
		$this->targetFragment = $targetFragment;
	}

	public function getHTMLFragments($gridField) {
		$button = new GridField_FormAction(
			$gridField, 
			'deleteall', 
			_t('TableListField.DELETEALL', 'Delete All'),
			'deleteall', 
			null
		);
		$button->setAttribute('data-icon', 'cross-circle');
		$button->addExtraClass('no-ajax');
		return array(
			$this->targetFragment => '<p class="grid-csv-button">' . $button->Field() . '</p>',
		);
	}

	public function getActions($gridField) {
		return array('deleteall');
	}

	public function handleAction(GridField $gridField, $actionName, $arguments, $data) {
		if($actionName == 'deleteall') {
			return $this->handleDeleteAll($gridField);
		}
	}

	public function handleDeleteAll($gridField, $request = null) {
		//Add your own DELETE logic here !
		$items=$gridField->getList();
		foreach ($items as $item) {
			$item->delete();
		}
		Controller::curr()->redirectBack();
	}
}

Any help, much appreciated.