I am having the same problem as well. Also using QB Mac 2019. I also just updated OS to 10.14.3 and the problem still persists. Layout Designer crashes immediately upon launch. This is part of the problem report:
Exception Type: EXC_CRASH (Code Signature Invalid)
Exception Codes: [removed]000000, [removed]000000
Exception Note: EXC_CORPSE_NOTIFY

kernel messages:

Backtrace not available

Unknown thread crashed with X86 Thread State (64-bit):
rax: [removed]000000 rbx: [removed]000000 rcx: [removed]000000 rdx: [removed]000000
rdi: [removed]000000 rsi: [removed]000000 rbp: [removed]000000 rsp: 0x00007ffee0d37b58
r8: [removed]000000 r9: [removed]000000 r10: [removed]000000 r11: [removed]000000
r12: [removed]000000 r13: [removed]000000 r14: [removed]000000 r15: [removed]000000
rip: 0x000000011bba8000 rfl: [removed]000200 cr2: [removed]000000