[rtemodel] Fix nullptr exception in `RteCallback::ExpandString` #1566

Job	Run time
coverage	4m 0s
setup / config	1s
matrix_prep / matrix_prep	3s
build (macos-12, darwin, amd64, csolution, publicRepo, true, true)	2m 0s
build (macos-12, darwin, arm64, csolution, publicRepo, false, false)	3m 10s
build-swig (macos-12, darwin, amd64, csolution, publicRepo, true, true)	4m 43s
build (ubuntu-22.04, linux, amd64, csolution, always, true, true)	1m 38s
build-swig (macos-12, darwin, arm64, csolution, publicRepo, false, false)	5s
build (ubuntu-22.04, linux, arm64, csolution, always, false, false)	2m 20s
build-swig (ubuntu-22.04, linux, amd64, csolution, always, true, true)	2m 24s
build (windows-2019, windows, amd64, csolution.exe, always, true, false)	4m 6s
build (windows-2019, windows, arm64, csolution.exe, always, false, false)	4m 6s
build-swig (ubuntu-22.04, linux, arm64, csolution, always, false, false)	5s
build-swig (windows-2019, windows, amd64, csolution.exe, always, true, false)	4m 49s
unittest (macos-12, darwin, amd64, csolution, publicRepo, true, true)	4m 27s
build-swig (windows-2019, windows, arm64, csolution.exe, always, false, false)	7s
unittest (macos-12, darwin, arm64, csolution, publicRepo, false, false)	2m 58s
unittest (ubuntu-22.04, linux, amd64, csolution, always, true, true)	3m 1s
unittest (ubuntu-22.04, linux, arm64, csolution, always, false, false)	1m 58s
unittest (windows-2019, windows, amd64, csolution.exe, always, true, false)	7m 48s
unittest (windows-2019, windows, arm64, csolution.exe, always, false, false)	3m 36s
Publish Tests Results	3s
release	0s
	57m 28s

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