I build from UE source code and the editor starts after the splash screen displayed for a few seconds. Here is the log from Xcode:
=================================================================
Main Thread Checker: UI API called on a background thread: -[NSApplication setMainMenu:]
PID: 71972, TID: 480482, Thread name: (none), Queue name: com.apple.root.default-qos.overcommit, QoS: 0
Backtrace:
4 UnrealEditor-Slate.dylib 0x0000000114ed6303 _ZN13FSlateMacMenu15PostInitStartupEv + 275
5 UnrealEditor-ApplicationCore.dylib 0x000000010e82a73a _ZN27FMacPlatformApplicationMisc8PostInitEv + 186
6 UnrealEditor 0x00000001047801e9 _ZN11FEngineLoop24PreInitPostStartupScreenEPKDs + 8473
7 UnrealEditor 0x000000010476ffb9 _Z11GuardedMainPKDs + 601
8 UnrealEditor 0x00000001047887c9 -[UEAppDelegate runGameThread:] + 377
9 UnrealEditor-Core.dylib 0x000000010f700dac -[FCocoaGameThread main] + 236
10 Foundation 0x00007ff801b34f33 __NSThread__start__ + 1007
11 libsystem_pthread.dylib 0x00000001179a28d7 _pthread_start + 99
12 libsystem_pthread.dylib 0x00000001179aac33 thread_start + 15
Main Thread Checker: UI API called on a background thread: -[NSApplication setMainMenu:]
PID: 71972, TID: 480482, Thread name: (none), Queue name: com.apple.root.default-qos.overcommit, QoS: 0
Backtrace:
4 UnrealEditor-Slate.dylib 0x0000000114ed6303 _ZN13FSlateMacMenu15PostInitStartupEv + 275
5 UnrealEditor-ApplicationCore.dylib 0x000000010e82a73a _ZN27FMacPlatformApplicationMisc8PostInitEv + 186
6 UnrealEditor 0x00000001047801e9 _ZN11FEngineLoop24PreInitPostStartupScreenEPKDs + 8473
7 UnrealEditor 0x000000010476ffb9 _Z11GuardedMainPKDs + 601
8 UnrealEditor 0x00000001047887c9 -[UEAppDelegate runGameThread:] + 377
9 UnrealEditor-Core.dylib 0x000000010f700dac -[FCocoaGameThread main] + 236
10 Foundation 0x00007ff801b34f33 __NSThread__start__ + 1007
11 libsystem_pthread.dylib 0x00000001179a28d7 _pthread_start + 99
12 libsystem_pthread.dylib 0x00000001179aac33 thread_start + 15
*** Assertion failure in -[FCocoaMenu _setMenuName:], NSMenu.m:795
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'API misuse: setting the main menu on a non-main thread. Main menu contents should only be modified from the main thread.'
*** First throw call stack:
(
0 CoreFoundation 0x00007ff800c0ab8e __exceptionPreprocess + 242
1 libobjc.A.dylib 0x00007ff8006f0f12 objc_exception_throw + 62
2 Foundation 0x00007ff801bfba31 -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 362
3 AppKit 0x00007ff804432a71 -[NSMenu _setMenuName:] + 859
4 AppKit 0x00007ff804448de9 -[NSApplication setMainMenu:] + 427
5 UnrealEditor-Slate.dylib 0x0000000114ed6303 _ZN13FSlateMacMenu15PostInitStartupEv + 275
6 UnrealEditor-ApplicationCore.dylib 0x000000010e82a73a _ZN27FMacPlatformApplicationMisc8PostInitEv + 186
7 UnrealEditor 0x00000001047801e9 _ZN11FEngineLoop24PreInitPostStartupScreenEPKDs + 8473
8 UnrealEditor 0x000000010476ffb9 _Z11GuardedMainPKDs + 601
9 UnrealEditor 0x00000001047887c9 -[UEAppDelegate runGameThread:] + 377
10 UnrealEditor-Core.dylib 0x000000010f700dac -[FCocoaGameThread main] + 236
11 Foundation 0x00007ff801b34f33 __NSThread__start__ + 1007
12 libsystem_pthread.dylib 0x00000001179a28d7 _pthread_start + 99
13 libsystem_pthread.dylib 0x00000001179aac33 thread_start + 15
)
An uncaught exception was raised
API misuse: setting the main menu on a non-main thread. Main menu contents should only be modified from the main thread.
(
0 CoreFoundation 0x00007ff800c0ab8e __exceptionPreprocess + 242
1 libobjc.A.dylib 0x00007ff8006f0f12 objc_exception_throw + 62
2 Foundation 0x00007ff801bfba31 -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 362
3 AppKit 0x00007ff804432a71 -[NSMenu _setMenuName:] + 859
4 AppKit 0x00007ff804448de9 -[NSApplication setMainMenu:] + 427
5 UnrealEditor-Slate.dylib 0x0000000114ed6303 _ZN13FSlateMacMenu15PostInitStartupEv + 275
6 UnrealEditor-ApplicationCore.dylib 0x000000010e82a73a _ZN27FMacPlatformApplicationMisc8PostInitEv + 186
7 UnrealEditor 0x00000001047801e9 _ZN11FEngineLoop24PreInitPostStartupScreenEPKDs + 8473
8 UnrealEditor 0x000000010476ffb9 _Z11GuardedMainPKDs + 601
9 UnrealEditor 0x00000001047887c9 -[UEAppDelegate runGameThread:] + 377
10 UnrealEditor-Core.dylib 0x000000010f700dac -[FCocoaGameThread main] + 236
11 Foundation 0x00007ff801b34f33 __NSThread__start__ + 1007
12 libsystem_pthread.dylib 0x00000001179a28d7 _pthread_start + 99
13 libsystem_pthread.dylib 0x00000001179aac33 thread_start + 15
)
FAULT: NSInternalInconsistencyException: API misuse: setting the main menu on a non-main thread. Main menu contents should only be modified from the main thread.; {
NSAssertFile = "NSMenu.m";
NSAssertLine = 795;
}
libc++abi: terminating due to uncaught exception of type NSException
Message from debugger: Terminated due to signal 6
