gw-cache-buster.php: Fixed fatal error from get_default_theme function.

Author: barthc

gravity-forms/gw-cache-buster.php

@@ -72,7 +72,7 @@ public function set_form_styles( $form, $style_settings, $form_theme ) {
 			unset( $form_styles['theme'] );
 		}
 		$form['styles'] = GFFormDisplay::get_form_styles( $form_styles );
-		$form['theme'] = ! empty( $form_theme ) ? $form_theme : GFForms::get_default_theme();
+		$form['theme']  = $form_theme;
 		return $form;
 	}