break_on_function_many_child_isolates_test failing on ubsan bot

[liamappelbe]

break_on_function_many_child_isolates_test is failing on the ubsan bot. It's a NPE at jit/compiler.cc:246

Logs: https://dart-ci.appspot.com/log/vm-kernel-ubsan-linux-release-x64/dartk-ubsan-linux-release-x64/1038/service_2/break_on_function_many_child_isolates_test/service

This test has previously been flaky, but the new failure looks different. Might be related to isolate groups?

[dcharkes]

Another one today: https://dart-ci.appspot.com/log/vm-kernel-ubsan-linux-release-x64/dartk-ubsan-linux-release-x64/1046/service_2/break_on_function_many_child_isolates_test/service

cc @bkonyi service protocol
cc @aam @mkustermann isolate groups

[mkustermann]

Seems like a benign, trivial issue. Made cl/215547