Running the launcher produces a suitable Minecraft app icon on the Mac task bar. Selecting a particular dev release, logging in and clicking play, currently reliably causes a 'launcher error 139', 'Invalid memory access of location 0x0 rip=0x0'. This is already logged as MCL-90, and is a known problem with Mac Os 10.5.8.

But in addition, after the error, the launcher itself loses its icon, even though it's still running OK.

To reproduce, trying adding and running a borked version of Minecraft itself to the launcher; on Mac OS 10.5.8 every version does this; on other OSes you'd probably need to deliberately break something.