Remove unnecessary references to colours

They were blocking the upgrade to Flutter 2.10

Author: luckyrat

lib/src/models/node.dart

@@ -1,5 +1,4 @@
 import 'dart:convert';
-import 'dart:ui';
 
 import 'package:flutter/widgets.dart';
 

lib/src/tree_view_theme.dart

@@ -132,7 +132,6 @@ class TreeViewTheme {
   /// replaced by the non-null parameters of the given icon theme. If the given
   /// [TreeViewTheme] is null, simply returns this theme.
   TreeViewTheme merge(TreeViewTheme other) {
-    if (other == null) return this;
     return copyWith(
         colorScheme: other.colorScheme,
         levelPadding: other.levelPadding,