Dart

Fix getDefaultFile method to use firstOrNull #265

Job	Run time
build (packages/lw_file_system)	53s
build (packages/lw_file_system_api)	47s
build (packages/consoler)	46s
build (packages/flutter_localizer)	51s
build (packages/lw_sysapi)	58s
build (packages/localizer)	46s
build (packages/dart_leap)	45s
build (packages/material_leap)	1m 2s
build (packages/networker/networker_socket)	48s
build (packages/networker/networker_crypto)	43s
build (packages/networker/networker)	48s
build (packages/networker/networker_s5)	48s
build (packages/networker/networker_rtc)	46s
build (packages/generate_leap)	43s
	11m 24s

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