Export crashes whole system

• My whole iMac system crashes when I use a particular share setting.

• This seems to happen only when I use the Apple 720 setting:

no chapters

roles - english captions

• I have exported without 'roles' and it shares fine without crashing the system.
• The Apple 1080 setting works fine also - but the file size is better for me at 720.
• It also does not crash the system when I use other export settings

WORKAROUND

• My workaround was to put the whole library on a firewire drive and open it on my MacBookPro which is on the same OS and version of FCPX that my iMac is running (Mojave with FCPX 10.4.8).
• This project exports fine on my MacBookPro using the same Apple 720 export setting that crashed the whole system on my iMac.

POSSIBLE SOLUTION

My theory is that the export setting Apple Devices 720 is corrupted on the iMac .

The Apple Devices 720 setting doesn't seem to me to be an actual compressor setting so I am stumped.

When I try to duplicate the Apple Devices HD setting below the option to add roles is not present when I add this setting to FCPX.

My question is:

How do i move the Apple Devices 720 setting from my MacBook Pro to my iMac?

Any other insight greatly appreciated.