There is still code for handling a nullptr being returned by BarrierSet::barrier_set()->barrier_set_nmethod(). Judging by the BarrierSet::BarrierSet constructor[2], and the definition of select_barrier_set_nmethod[3], it's impossible for barrier_set_nmethod() to ever return nuillptr.

[1] https://github.com/openjdk/jdk/blob/7c22b814d670deda6c2bb93b1e150975c27a165f/src/hotspot/share/jvmci/jvmciCompilerToVMInit.cpp#L168
[2] https://github.com/openjdk/jdk/blob/da2b4f0749dffc99fa42c7311fbc74231af273bd/src/hotspot/share/gc/shared/barrierSet.cpp#L73
[3] https://github.com/openjdk/jdk/blob/da2b4f0749dffc99fa42c7311fbc74231af273bd/src/hotspot/share/gc/shared/barrierSet.cpp#L54