PageCreateDialog: prevent tmp using actual page

Fixes #6640

src/Panel/PageCreateDialog.php

@@ -249,7 +249,7 @@ public function load(): array
 	public function model(): Page
 	{
 		return $this->model ??= Page::factory([
-			'slug'     => 'new',
+			'slug'     => '__new__',
 			'template' => $this->template,
 			'model'    => $this->template,
 			'parent'   => $this->parent instanceof Page ? $this->parent : null
@@ -267,12 +267,7 @@ public function resolveFieldTemplates(array $input): array
 
 		// create temporary page object
 		// to resolve the template strings
-		$page = new Page([
-			'slug'     => 'tmp',
-			'template' => $this->template,
-			'parent'   => $this->model(),
-			'content'  => $input
-		]);
+		$page = $this->model()->clone(['content' => $input]);
 
 		if (is_string($title)) {
 			$input['title'] = $page->toSafeString($title);