[SOLVED] not able to save. dark screen while recording with QuickTime

Hi, I am experiencing a strage Dorico 2 behavior: I am not able to save this particular file. Please see a short video:

Additionally: I noticed a strange macOS (a last official version 10.13.5) behavior while recording this video with QuickTime. When I quit Dorico a screen goes dark for a second, when I start Dorico again, the screen stays dark till the end of my recording. I tried it several times - each time the same result. Recording a gif with Gifox App - the same result as well. This problem doesn’t occur, while recording other applications. It is related somehow exlusively to Dorico. I DO NOT experience this problem while working with Dorico normally. Even when I start or quit it.
Polnisches Lied.dorico.zip (363 KB)

The problem is not that Dorico won’t save your project: the problem is that it shouldn’t really allow you to appear to delete that bar in the first place. If you switch on the Tempo signposts you’ve switched off, you’ll see that you have a tempo event in the final bar of the project, and deleting that bar doesn’t delete that tempo for reasons I’m unsure of (and will check with the team). When you reopen the project, that bar is recreated because that tempo is still there. Delete the tempo event, then delete the bar, then save and reopen your project.

Thank you Daniel, your answer was very helpful. Now I understand, why I couldn’t use in this case “trim flow” as well.