I just updated to Hugin 2018.0.0 on macOS 10.13.5, and when I try to stitch the project using the Stitch! button on the Stitch pane, nothing happens.
I have Settings > Stitch > Processor set to Hugin_stitch_project; when I choose PTBatcherGUI instead the Stitch! button has the expected effect of opening PTBatcherGUI.
If I open /Applications/Hugin/HuginStitchProject.app directly (by double-clicking on its icon) I can select my .pto file and it will stitch it without problems.
Why won't the Stitch! button work with HuginStitchProject?