Unclosed markdown tags in subtitle lead to Pango breakage

[aljelly]

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For example, try the following in a new note:

# My title
break_pango ()

# My title
I like **corrections

Markdown formatting without closing tags converts to HTML without closing tags, breaking Pango:

(Notes-Up:21977): Gtk-WARNING **: 03:02:46.508:
Failed to set text 'Add to Page.vala, new<i>page (), line 163'
from markup due to error parsing markup:
Error on line 1 char 58: Element 'markup' was closed, but the currently open element is 'i'

This prevents the title and preview labels from updating until the error is fixed.

If you restart Notes-Up while leaving the note contents as it is, the preview label doesn't show until you remove or fix the text.