Document can't be exported when inline formulas are in other blocks than paragraph

### Description

Whenever a formula is present in a block that's not a paragraph, e.g. a list or a headline, exporting fails without showing an error.

- [x] Can you reproduce the issue?

### Steps to reproduce
Try to export
```md
# $1+1=2$
```

### Versions

- Mark Text: 15.1
- Operating system: macOS 10.15
