The solution was to enable "Disable Library Validation" for the extension - find it in the Signing & Capabilities tab of the target settings.

Additional Info: I'm using Final Cut Pro Trial. When I'm attached to the extension process and pause it this is the main thread stack trace: Thread 1 Queue : com.apple.main-thread (serial) #0 0x00007ff81d6b497a in mach_msg_trap () #1 0x00007ff81d6b4ce8 in mach_msg () #2 0x00007ff81d7b836d in __CFRunLoopServiceMachPort () #3 0x00007ff81d7b69f8 in __CFRunLoopRun () #4 0x00007ff81d7b5e3c in CFRunLoopRunSpecific () #5 0x00007ff8264655e6 in RunCurrentEventLoopInMode () #6 0x00007ff82646534a in ReceiveNextEventCommon () #7 0x00007ff8264650e5 in _BlockUntilNextEventMatchingListInModeWithFilter () #8 0x00007ff8201effad in _DPSNextEvent () #9 0x00007ff8201ee66a in -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] () #10 0x00007ff824d2d1c2 in __75-[NSViewServiceApplication nextEventMatchingMask:untilDate:inMode:dequeue:]_block_invoke () #11 0x00007ff824d2d003 in -[NSViewServiceApplication _withToxicEventMonitorPerform:] () #12 0x00007ff824d1abf3 in -[NSViewServiceApplication nextEventMatchingMask:untilDate:inMode:dequeue:] () #13 0x00007ff8201e0d19 in -[NSApplication run] () #14 0x00007ff8201b4c97 in NSApplicationMain () #15 0x00007ff81d439874 in _xpc_objc_main () #16 0x00007ff81d439239 in xpc_main () #17 0x00007ff81e6362fc in -[NSXPCListener resume] () #18 0x00007ff82901d7d6 in ___lldb_unnamed_symbol834 () #19 0x00007ff82901d45c in ___lldb_unnamed_symbol830 () #20 0x00007ff82901dba5 in ___lldb_unnamed_symbol838 () #21 0x00007ff830642385 in EXExtensionMain () #22 0x00007ff81e6a0918 in NSExtensionMain () #23 0x0000000115f8752e in start ()

Solved by adding libc++.tbd to Link Binary with Libraries in the Build Phases of the Widget Extension.