Check for valid DynamicFontData before duplicate

(cherry picked from commit ae2bc66eaee87bf27b6da775672b5a9737e44f64)

editor/plugins/editor_preview_plugins.cpp

@@ -841,7 +841,9 @@ Ref<Texture> EditorFontPreviewPlugin::generate_from_path(const String &p_path, c
 	sampled_font.instance();
 	if (res->is_class("DynamicFont")) {
 		Ref<DynamicFont> font = res;
-		sampled_font->set_font_data(font->get_font_data()->duplicate());
+		if (font->get_font_data().is_valid()) {
+			sampled_font->set_font_data(font->get_font_data()->duplicate());
+		}
 		for (int i = 0; i < font->get_fallback_count(); i++) {
 			sampled_font->add_fallback(font->get_fallback(i)->duplicate());
 		}