Revert "Don't set the listener null when disconnecting."

This reverts commit 6ccb2a514cb80f2a26978e86c0f46c892e7257cf.

The java version of the library has different behavior here
for some reason, but we explicitly want to prevent a disconnected
listener from being called, regardless of whether a dispatch
was in progress
change SignalTest.testRemoveDuringDispatch to match intended behavior