Steps to reproduce:
Paste the following wikitext in a page or simply use the parsermigration-edit , and click preview:
<i> <b>foo<b>k <b>foo</i>
[WjpdFQpAICkAAKBp10gAAAAY] 2017-12-20 12:52:37: Fatal exception of type "RemexHtml\TreeBuilder\TreeBuilderError"
Note the space behind the <i> . That's a pre block, it also seems to happen with a div and perhaps other tags.