setting string attribute of a tag crashes after calling tag.parsed_contents

Example below:

```
import wikitextparser as wtp

wikicode = wtp.parse("<ref>a [reference]</ref>")
tag = wikicode.get_tags()[0]

tag.parsed_contents
tag.string = "<ref>a modified [reference]</ref>"
```

Commenting out the call `tag.parsed_contents` produces expected behaviour. Then content of the tag doesn't seem to matter.