Extra content at the end of the document

### Issue Description

After saving a rather complex diagram, I get the following message:

```
This page contains the following errors:
error on line 1 at column 168258: Extra content at the end of the document
Below is a rendering of the page up to the first error.
```
The diagram is shown on the wiki page. But it can't be edited anymore.

Best regards
Juergen