Hi @Vasili
If get_field_ID() (not a field, but a WP variable) is false, then WP does not have a global $post at the time of your widget.
This explains why ACF is not able to load.
Where in the template is this widget loaded?
On what page is the widget loaded, or perhaps all?
You will need to find a way to get a post_id (numeric value) for ACF to load the value from.
Thanks
E
