Error after upgrading from 5.1.3 to 6.0.0

Initial page loads and the dev tools are active, but seem buggy. For example if I click on the route diagram, it flashes the "real" diagram and then reverts to a single dot with "Root". As soon as I try to navigate, I get:

```
Application Error
TypeError: Cannot read properties of undefined (reading 'add')
at http://localhost:4200/node_modules/.vite/deps/react-router-devtools_client.js?v=e1651124:12939:44
at http://localhost:4200/node_modules/.vite/deps/react-router-devtools_client.js?v=e1651124:12941:5
at http://localhost:4200/node_modules/.vite/deps/react-router-devtools_client.js?v=e1651124:12941:5
at http://localhost:4200/node_modules/.vite/deps/react-router-devtools_client.js?v=e1651124:12941:5
at http://localhost:4200/node_modules/.vite/deps/react-router-devtools_client.js?v=e1651124:12941:5
at http://localhost:4200/node_modules/.vite/deps/react-router-devtools_client.js?v=e1651124:12941:5
at http://localhost:4200/node_modules/.vite/deps/react-router-devtools_client.js?v=e1651124:12941:5
at http://localhost:4200/node_modules/.vite/deps/react-router-devtools_client.js?v=e1651124:12951:13
at http://localhost:4200/node_modules/.vite/deps/react-router-devtools_client.js?v=e1651124:12929:5
at http://localhost:4200/node_modules/.vite/deps/react-router-devtools_client.js?v=e1651124:12932:7
```

Revert to 5.1.3 and everything works fine again.

React Router 7.10.1
Vite 7.2.7
React 18.3.1

Spa mode

在一个使用 SPA mode 的 React Router 7.10.1、Vite 7.2.7、React 18.3.1 应用中重现该失败，并比较使用 devtools 5.1.3 和 6.0.0 时的行为。从 stack trace 中报告的 devtools 客户端代码开始，检查 route-diagram 和导航路径。完成标准是图表保持正确，且导航不再引发 undefined 的“add”错误。

由索引模型根据 Issue 内容生成。