Needs Output Buffering

This plugin is great and is really close to what I needed. I did have to modify it a little bit to make it work. The current plugin uses dynamic_sidebar() function which is outputting content outside of your $value variable and isn’t returned with the function.

By adding ob_start() and ob_get_contents you can save this output and return it properly within your div. (If you look now, the <div id=”secondary”> gets displayed after the sidebar)

Here is the final code change to make this work so that ACF functions get_field (returns the html) and the_field (displays the html) work properly.

<br />
function format_value_for_api($value, $post_id, $field)<br />
{<br />
	$value = '';<br />
	if ( is_active_sidebar( $field['value'] ) ) :<br />
		$value .= '<div id="secondary" class="widget-area" role="complementary">';<br />
			ob_start();<br />
			dynamic_sidebar( $field['value'] );<br />
			$value .= ob_get_contents();<br />
			ob_end_clean();<br />
		$value .= '</div><!-- #secondary -->';<br />
	endif;<br />
	return $value;<br />
}<br />

https://wordpress.org/plugins/advanced-custom-fields-widget-area-field/