Widget to show posts in the sidebar basing on its IDs

Try to replace

'post__in' => array($instance['postsToShow']),

with

'post__in' => explode(",", $instance['postsToShow']),

to feed the post__in with an array.

Your current array looks like

array("123,456,789")

but should be like

array("123", "456", "789")

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