Hello. Had a nightmare with this issue.. this is a Prestashop 1.7.5.0 fatal bug, the only reason why this isn't exploding is because there aren't many shops moving to 1.7.5.0
Anyway, 1.7.5.1 seems to correct this. If upgrade is not an option, just replace the following folders with the ones from 1.7.5.1:
/public_html/src/PrestaShopBundle/Controller/Admin
/public_html/src/PrestaShopBundle/Resources/views/Admin
Also, after you replace folders the image editing form seems to work only when DEBUG is active.