Dotty

Add error-checking when fetching rhs of trees from TASTy #34538

Job	Run time
test_non_bootstrapped	0s
test	58m 58s
test_scala2_library_tasty	0s
test_windows_fast	18m 3s
MiMa	10m 50s
community_build_a	1h 19m 19s
community_build_b	1h 47m 22s
test_windows_full	0s
build-msi-package	0s
community_build_c	1h 3m 17s
test_java8	0s
test_sbt	18m 28s
build-sdk-package / build	7m 57s
test-msi-package	0s
build-chocolatey-package / build	21s
test-chocolatey-package	0s
publish_nightly	0s
publish_release	0s
nightly_documentation	0s
open_issue_on_failure	0s
	6h 4m 35s

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