The About Inkscape dialog cannot be closed on macOS

Steps to reproduce:

• open Inkscape
• Click on Help -> About Inkscape
• Click on the red close (x) button of the About Inkscape dialog

What happened?

(Duplicate of #631 (moved)) The About Inkscape dialog stays on the screen

What should have happened?

The About Inkscape dialog should have disappeared

Inkscape Version and Operating System:

• Inkscape Version: 1.0alpha2 863a58d
• Operating System: macOS
• Operating System version: 10.13