iOS 13 app crash on exception with reason (null)

My app is crashing with a bizzare error on beta5. Here are a little history of this:

beta1, 2, 3 - working perfectly

beta4 - unable to compile

beta5 - can build, but crash on launch, details below.

My code is unchanged since beta3 - so definitely not something I've modified since then.

Here is the error:

2019-08-06 22:47:11.181717+0800 My App Mobile[19708:876341] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '(null)'
*** First throw call stack:
Call stacks here. I don't think this is neccessary

I've placed some breakpoint, it crashed after running:

application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?)

and:

application(_ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession, options: UIScene.ConnectionOptions)