Magento 2 required parameter 'theme_dir' was not passed
If you are getting this error when trying to update product from admin or programmatically then it's because of you have deleted your custom theme directory but didn't delete from admin settings.
Follow the below steps to solve this issue in Magento 2.
Step 1: Go to Magento Admin
Step 2: Move to CONTENT > Themes
Step 3: Remove theme which directory is already deleted by you.
Step 4: Go to CONTENT > Configuration
Step 5: Assign the proper theme to your website store.