I’ve had a couple of similar experiences. I usually build and test my apps on iOS using XCode, and rely on the console there for error messages, but I was occasionally getting to the stage where my app would not start properly, but I wouldn’t see anything in XCode.
I found that if I turned on Web Inspector on my device, and then used Safari to debug my running app, the Safari developer console would give me a lot more debug information than XCode, and this has helped me to debug things like variables being null when a value was expected etc.
I normally dislike using the Safari developer tools as you cannot actually start the tool until the app is ALREADY running on the device, thus reducing the effectiveness of debugging startup code, but I learned a great tip on this forum, which is to enter the command
in the Safari developer console to restart your app so you can do start up debugging… Hope this helps.