Problem:

Validation of the featured image in the Toolset Forms was triggering an error.

Solution:

The issue was that the PHP function 'getimagesize' expects the param to be an actual object and in the error log you shared this parameter is empty.

I deducted that this happened when you try to submit the form without the featured image, then getimagesize will try to check the size of null and error out.

I added a check to see if the field _featured_image is set before checking for the size and that fixed the issue.