I wrote recently that there were no known problems with our apps on macOS Sierra. It turned out I had overlooked one subtle aspect of the update, which did have a negative impact on two Red Sweater apps.
Automatic window tabs grant the ability to almost all apps on the Mac to organize multiple documents in tabs, the way you have been able to do for a long time in Safari.
Unfortunately this functionality doesn’t lend itself perfectly to MarsEdit or Black Ink. In MarsEdit, it permits tabbing in the main window, where it doesn’t make much sense, and also creates a confusing arrangement of tabs in document windows, because the preview window and main document each get their own separate tabs. In Black Ink, the issue is worse, because enabling tabs and pushing the “+” button causes a crash in the app.
I have updated these apps to disable automatic window tabbing functionality. “Check for Updates” or download the latest version from the Black Ink or MarsEdit home pages. Updates for the Mac App Store are awaiting review by Apple.
I’ll be looking into the tabbing functionality in the future to see if I can adapt it to the apps without causing unwanted side-effects.