Sidebar widget plugin makes TinyMCE disappear

I’m writing a plugin… ok, let me start again. Im copying and pasting code chunks from the “recent entries” sidebar widget (/wp-includes/widgets.php) in order to make a widget that shows the latest posts for a given category.

It does what it should, but when I activate it, the TinyMCE toolbar disappears from the places where it should show up.

The plugin is available here.

Does anyone have an idea of why this might be happening?