Filtering the main loop, not the sidebar

Hi!

I’m using the following code to prevent posts in specific categories from appearing on front page:

function front_page_excludes($query) {
	if($query->is_front_page) {
		$query->set(
			'category__not_in',
			array(1,2)
		);
	}
	return $query;
}
add_filter('pre_get_posts', 'front_page_excludes');

There’s a problem with this code, though. It’s affecting the recent posts widget on the sidebar, too, and I just want it to filter just the main loop, not the sidebar.

Any ideas?

Thanks in advance