Hm, at least we’re getting closer step-by-step.
Seems like it dies inside mbed_die with a fatal error (does the LED blink periodically?) or hangs in the radio initialization routines, possible at a SPI transfer. If you have a PIO Plus (trial), try debugging it stepwise [or follow my tutorial]. Also use a firmware with the added flag -ggdb to your build_flags for debug symbols.
Double check if the used connection pins for the constructor (SX1272Lib - Driver for the SX1272 RF Transceiver | Mbed Line 70 and SX1272Lib - Driver for the SX1272 RF Transceiver | Mbed Line 92) are correct for your board (which should be here [careful, upside down] and here)
I had a RFM95 radio attached to my Nucleo-L152 and got until a “RX Timeout”.