Steps to reproduce:
1. Open Unity on any version of 5.6 or above.
2. In the top menu bar go to Help -> Software licenses
3. Nothing happens

Expected result: Information about Software licenses is presented in legal.txt
Actual result: Nothing happens

Reproduced on: 2017.2.0a3, 2017.1.0b1, 2017.1.0b7, 2017.1.0b9, 5.6.1.p4, 5.6.0f3
Tested on:
MacOS Sierra Version 10.12.5
OS X El Capitan Version 10.11.6

Note: one time 2017.1.0b9 managed to launch the legal.txt file, but when it was re-launched, it wouldn't work anymore.
Note: 5.5 did not have Software licenses under Help menu