So, I tried to reproduce the error you were experiencing and I was able to make my preview window stop rendering. I typed a bit and mussed with the markdown and was able to fix it. It seems like it's a problem with the markdown parser when using reference-style links.
I know it's no solution, but I prefer to manually enter my own links instead of using the toolbar, since Discourse defaults to using reference-style linking. When adding one or two discrete links in a block of text, reference-style links feel wrong to me. I can see using them for writing documentation or something where the same link will be referred to multiple times, but for non-tutorial forum posts, they seem overkill.
In case you were too important to read the above links, here's the gist of inline links.
To create an inline link, use a set of regular parentheses immediately after the link text’s closing square bracket. Inside the parentheses, put the URL where you want the link to point, along with an optional title for the link, surrounded in quotes. For example:
This is [an example](http://example.com/ "Title") inline link.
[This link](http://example.net/) has no title attribute.
<p>This is <a href="http://example.com/" title="Title"> an example</a> inline link.</p>
<p><a href="http://example.net/">This link</a> has no title attribute.</p>