Cannot read properties of null (reading 'getAttribute')

```umo-viewer.js 
const P = i.value.querySelector(".umo-page-content").getAttribute("data-katex-style");
```



will produce error :
```
umo-viewer.js:27024  Uncaught (in promise) TypeError: Cannot read properties of null (reading 'getAttribute')
    at m10 (umo-viewer.js:27024:58)
```
