Hi,
There was a similar question some time ago, have a look here: GtkSourceView with a custom highlighting engine for a custom language
You will have to make sure to disable the gtksourceview own highlighting (set GtkSource.Buffer:highlight-syntax to FALSE) before applying your custom tags.
I really don’t see where that could come from… Try setting the env variable G_DEBUG=fatal-criticals and launch the app in a debugger to get a call stack.