Fatal error when creating listen server on an instance with a PendingNetGame

This seems to occur because the PendingNetGame is not destroyed when the instance browses to the new level to start listening.
In UEngine::LoadMap, UWorld::SetGameMode will create the game mode, provided the NetMode of the world is not NM_Client.
This is done before the call to UWorld::Listen, so when attempting to create the game mode, the listen server instance does not have a game NetDriver yet. When IsNetMode(NM_Client) is called here, UWorld::AttemptDeriveFromURL will return NM_Client, as there is a PendingNetGame on the world without the "listen" option in its URL.

On a standalone editor instance, use the "open 127.0.0.1" command to attempt to connect to a server. (A PendingNetGame will be created, but it will not connect since there is no server.)
Then use "open <MapName>?listen" to create a listen server.
Observe the fatal error "Couldn't spawn player..." hit in UEngine::LoadMap.