When I open the launcher it closes nearly instantly again. No window is opened. 

I downloaded the latest version of the launcher and Minecraft again and installed them and restarted my computer (Mac OS 10.11.6). Still not opening.

Went to the app and clicked on "Show Package Contents" and launched the  launcher. 
This then gives the following error:
"Unable to load nib file: NativeUpdater, exiting"

Please find attached a recording of the process.