Homepage.php

<?php

/**
 * Class HomePage
 *
 * @method ContentBlockSection TopContentBlockSection()
 * @method ContentBlockSection MiddleContentBlockSection()
 * @method ContentBlockSection BottomContentBlockSection()
 */
class HomePage extends Page {

	private static $db = [
		'HeaderText' => 'HTMLText'
	];

	private static $belongs_to = [
		'TopContentBlockSection'    => 'ContentBlockSection.Page',
		'MiddleContentBlockSection' => 'ContentBlockSection.Page',
		'BottomContentBlockSection' => 'ContentBlockSection.Page'
	];

	public function getCMSFields() {

		$this->beforeUpdateCMSFields(function (FieldList $fields) {
			$fields->addFieldToTab('Root.Main', HtmlEditorField::create('HeaderText')->setRows(5));

			if ($this->Exists()) {

				if (!$this->TopContentBlockSection()->exists()) {
					$this->TopContentBlockSection()->Title = 'Top Content Blocks';
				}

				if (!$this->MiddleContentBlockSection()->exists()) {
					$this->MiddleContentBlockSection()->Title = 'Middle Content Blocks';
				}

				if (!$this->BottomContentBlockSection()->exists()) {
					$this->BottomContentBlockSection()->Title = 'Bottom Content Blocks';
				}

				$TopContentBlocksConfig = new GridFieldConfig_RelationEditor();
				$TopContentBlocksConfig->addComponent(new GridFieldSortableRows('SortOrder'));

				$fields->addFieldsToTab('Root.TopContentBlocks', [
					TextField::create('TopContentBlockSectionTitle', 'Title', $this->TopContentBlockSection()->Title),
					GridField::create("TopContentBlocksGridField", "Content Blocks", $this->TopContentBlockSection()->Blocks(), $TopContentBlocksConfig)
				]);

				$MiddleContentBlocksConfig = new GridFieldConfig_RelationEditor();
				$MiddleContentBlocksConfig->addComponent(new GridFieldSortableRows('SortOrder'));

				$fields->addFieldsToTab('Root.MiddleContentBlocks', [
					TextField::create('MiddleContentBlockSectionTitle', 'Title', $this->MiddleContentBlockSection()->Title),
					GridField::create("MiddleContentBlocksGridField", "Content Blocks", $this->MiddleContentBlockSection()->Blocks(), $MiddleContentBlocksConfig)
				]);

				$BottomContentBlocksConfig = new GridFieldConfig_RelationEditor();
				$BottomContentBlocksConfig->addComponent(new GridFieldSortableRows('SortOrder'));

				$fields->addFieldsToTab('Root.BottomContentBlocks', [
					TextField::create('BottomContentBlockSectionTitle', 'Title', $this->BottomContentBlockSection()->Title),
					GridField::create("BottomContentBlocksGridField", "Content Blocks", $this->BottomContentBlockSection()->Blocks(), $BottomContentBlocksConfig)
				]);
			}
		});

		$fields = parent::getCMSFields();

		return $fields;
	}

	public function onBeforeWrite() {
		parent::onBeforeWrite();

		$topContentBlockSection    = $this->TopContentBlockSection();
		$middleContentBlockSection = $this->MiddleContentBlockSection();
		$bottomContentBlockSection = $this->BottomContentBlockSection();

		sd($this->TopContentBlockSectionTitle); // NULL :( 

		$topContentBlockSection->Title    = $this->TopContentBlockSectionTitle;
		$middleContentBlockSection->Title = $this->MiddleContentBlockSectionTitle;
		$bottomContentBlockSection->Title = $this->BottomContentBlockSectionTitle;

		$topContentBlockSection->write();
		$middleContentBlockSection->write();
		$bottomContentBlockSection->write();

	}
}

class HomePage_Controller extends Page_Controller {

}