After connecting you must use this block, only once, like this:
This tells that you want to recieve messages. In my example app, the micro:bit would only reply to incoming messages, and I found out later, that you have to sent RegisterForBytes only once, otherwise you will get the message many times. But in your case you never send it at all, and therefore there is no message when you press A.
For reference I attach here the example files from my tutorial.
microbitRW_2.aia (304.6 KB)
microbit-MB_RW_2.hex (1.5 MB)
And maybe you have to ask permission if you are using Android > 10.
I can't agree more, ChrisWard! 
I tried the .aia and .hex files you attached and I enabled all permissions:
However, I still keep running into the following problem:
What you still can try is to load the .hex file in makecode and set no pairing required in the settings and then try again.
Hi,
I am encountering the same issue of the app crashing after i connect to the micro bit. It displays me this error code on my screen.
Thanks for any help.
Start with attaching the Blocks you used to Connect to the Micro:bit and your .hex code on the microbit
Hi,
I read your reply and I dont seem to understant what you ment by that. I am using the same file for the micro bit and in the mit app inventor as is it given on the App Inventor, Micro:Bit and UART tutorial.
Thanks for any more help.
Hi, I am not able to reply more precicely at this moment, but search this forum to find out how to upload your code to this forum.
If you used the exact files from the tutorial it should work, therefore something is different.
Sorry for the late reply i was on vacation and couldnt respond.
Perhaps I am using wrong files but it still doesnt work could you plese tell me which files exactly i must use?
Thanks you