All worked fine in iOS 11, suddenly in iOS 12 the project shows the UE4 splash screen and crashes. Nothing changed in the game to cause this. At a loss 
Had that too,
It works for me after i installed the latest Xcode & i had to start Xcode itself before i tried to use it with UE.
Xcode needs to install some stuff before it works, besides that, i also iTunes needs to install a update so that my iPhone with iOS12 works with iTunes again. I ignored the massage of iTunes before, because i got a similar msg. all the time when i connected my iPhone to the mac before but it always worked until iOS12.
Oddly, installing the latest xcode fixed the issue on some of my projects but not my major one, which still crashes in the splash screen. The Console throws a bunch of texture issues before eventually closing the process.
default 20:40:21.747230 -0700 BlankBase [2018.09.23-03.40.21:746][ 0]LogTexture: Error: Texture2D /Game/EternalTemple/Textures/T_Temple_Railing_01_N.T_Temple_Railing_01_N is unknown [raw type 0] which is not supported.
default 20:40:21.747287 -0700 BlankBase [2018.09.23-03.40.21:747][ 0]LogTexture: Error: Texture2D /Game/EternalTemple/Textures/T_Temple_Railing_01_M.T_Temple_Railing_01_M is unknown [raw type 0] which is not supported.
default 20:40:21.747331 -0700 BlankBase [2018.09.23-03.40.21:747][ 0]LogTexture: Error: Texture2D /Game/EternalTemple/Textures/T_Temple_Railing_01_BC.T_Temple_Railing_01_BC is unknown [raw type 0] which is not supported.
default 20:40:21.750029 -0700 BlankBase [2018.09.23-03.40.21:749][ 0]LogTexture: Error: Texture2D /Game/EternalTemple/Textures/T_Grass_01_N.T_Grass_01_N is unknown [raw type 0] which is not supported.
default 20:40:21.750118 -0700 BlankBase [2018.09.23-03.40.21:749][ 0]LogTexture: Error: Texture2D /Game/EternalTemple/Textures/T_Grass_01_BC.T_Grass_01_BC is unknown [raw type 0] which is not supported.
Did you check these textures making problems ?
I just experienced something similar after upgrading my iPad to iOS 12. After launching onto the device, the project would get stuck at the splash screen (no crash).
Packaging with Xcode running resolved the issue!
Experiencing the same problem here. Tried running the packaged project in older ios 11, works fine.On ios12 it crashes after splash screen. Installed the latest Xcode and kept it running while packaging - didnt work. Is there an ETA on when this issue will be fixed?