Fix function call error and null safety (#491)

codes/dart/chapter_tree/avl_tree.dart

@@ -143,7 +143,7 @@ class AVLTree {
         while (temp!.left != null) {
           temp = temp.left;
         }
-        node.right = removeHelper(node.right, temp!.val);
+        node.right = removeHelper(node.right, temp.val);
         node.val = temp.val;
       }
     }

codes/dart/chapter_tree/binary_search_tree.dart

@@ -135,7 +135,7 @@ void main() {
   print("\n查找到的节点对象为 $node，节点值 = ${node?.val}");
 
   /* 插入节点 */
-  node = insert(16);
+  insert(16);
   print("\n插入节点 16 后，二叉树为\n");
   printTree(getRoot());