We are currently aware of an issue where users tap "Done" and the screen blacks out and closes.
Our Dev team is investigating this issue and we hope to have a solution soon.
For the interim, so you don't lose all of your work, please tap on the diagonal arrows icon directly to the left of the "Done" button. This will remove all editing buttons and put your image by itself on the screen. From here, you can grab a screenshot of your image to post.
We do apologize for any inconvenience. Our Dev team has been working on this issue and has an update in testing. Once this update is confirmed, we will release an update to the iOS App Store... We appreciate your patience with this issue!