Avoid unused instantiation
diff --git a/lib/src/treebuilder.dart b/lib/src/treebuilder.dart
index 04c3a19..bc4c089 100644
--- a/lib/src/treebuilder.dart
+++ b/lib/src/treebuilder.dart
@@ -73,7 +73,7 @@
 class TreeBuilder {
   final String? defaultNamespace;
 
-  var document = Document();
+  late Document document;
 
   final List<Element> openElements = <Element>[];