NullReferenceException in PrioritySendDataCollection.Clear() #5566

We have two reports of a NullReferenceException in this method on macOS but it's not clear if the field(s) being accessed are null of if elements in the array are null.
The code is guarded by a Dbg.Assert but it's clear that a non-debug check is called for.
see Dbg.Assert(null != _dataToBeSent, "Serialized streams are not initialized");

See issue #4029 for repro steps.

Note that this is not repro'ing on machines available in-house.