Empty TextView when sending spoiler textblocks

[tfuxu]

When working on implementing spoilers, I've encountered this bug where when you send a textblock inside of a spoiler, a content handler (or presumably MarkdownViewer) creates an Gtk.TextView object normally intended for standard messages above that textblock, without any text inside.

Screenshots:

Not in spoiler:

In spoiler:

Minimal reproducer:

||```go
println("Text")
```||

Just send it from official client or gtkcord, and check using inspector if there is an unused TextView inside message row.