Fix observeEvent stack trace stripping #2668

Job	Run time
routine / routine	4m 24s
R-CMD-check / setup	10s
website / pkgdown	2m 41s
R-CMD-check / macOS-latest (4.4.2)	3m 46s
R-CMD-check / windows-latest (4.4.2)	4m 9s
R-CMD-check / ubuntu-latest (devel)	3m 24s
R-CMD-check / ubuntu-latest (4.4.2)	3m 5s
R-CMD-check / ubuntu-latest (4.3.3)	3m 33s
R-CMD-check / ubuntu-latest (4.2.3)	3m 33s
R-CMD-check / ubuntu-latest (4.1.3)	3m 22s
R-CMD-check / ubuntu-latest (4.0.5)	3m 10s
	35m 17s

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