Background DFU error NRF_ERROR_SVC_HANDLER_MISSING

hello

I used nrf52840 with SDK15.3.

1.I use the example in this ticket and adjusted the baud rate to 9600 without hardware flow control.After SES is compiled, an "NRF_ERROR_SVC_HANDLER_MISSING" error is reported in the background upgrade.After keil is compiled, an "Fatal error" is reported in the background upgrade.I have tried to change the serial port interrupt priority, but failed. How do I solve this problem?

https://devzone.nordicsemi.com/f/nordic-q-a/46994/background-dfu-application-source-code.

2.where can I find ble Background DFU application source code ?

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