You need to manually add the path to the modal article element. In the < fieldset > element just above the modal_article field type, add the following attribute:
addfieldpath="/administrator/components/com_content/models/fields"
So your final xml would look something like this:
<fieldset name="basic" addfieldpath="/administrator/components/com_content/models/fields">
<field name="article_id" type="modal_article" default="" label="Select an article" description="" />