front: fix a crash that would happen when editing a train #9895

Job	Run time
check_railjson_generator	1m 58s
check_generated_railjson_sync	23s
check_commits	6s
build (core-build, core)	16s
check_infra_schema_sync	18s
build (editoast, editoast-test)	13s
check_integration_tests	1m 34s
build (gateway-test, gateway-standalone, gateway-front)	1m 35s
build (front-build, front-tests, front-devel, front-nginx)	18s
check_osrd_schema	40s
check_toml	4s
build (osrdyne, osrdyne-test)	16s
Check final newline	7s
check_core	4m 3s
check_editoast_openapi	1m 29s
check_editoast_tests	5m 8s
check_editoast_lints	5m 50s
check_front	2m 31s
check_front_rtk_sync	1m 23s
check_gateway	2m 11s
check_osrdyne	2m 41s
integration_tests	6m 36s
	39m 40s

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