Problem: Form editor gets a blank (white) window when they launch File Upload Form editor inside the Google Sheet?
Solution: The issue can be resolved by logging out of multiple Google accounts and enabling third-party cookies for the Google subdomain.
Multiple Google Accounts
You are logged into multiple Google accounts and your default Google account is not the one that is connected to your File Upload Form.
Use the incognito mode workaround to get around this problem.
Enable Third-party Cookies
You may also see blank window if third-party cookies are disabled in your browser.
This video explains why some third-party cookies are necessary and how you can selectively allow cookies inside Google Chrome to use Google add-ons inside Sheets.
Open Chrome settings, go to the “Privacy and Security” section and expand
https://[*.]googleusercontent.comin the input box.
Add button and reload the Google Sheet to apply the changes.