GVFS.Mount will crash if it receives a ReleaseLock before mounting is complete

VFS For Git Version
1.0.19158.2

Callstack

"System.NullReferenceException: Object reference not set to an instance of an object.
   at GVFS.Mount.InProcessMount.HandleReleaseLockRequest(String messageBody, Connection connection) in E:\A\_work\593\s\GVFS\GVFS.Mount\InProcessMount.cs:line 343
   at GVFS.Common.NamedPipes.NamedPipeServer.HandleConnection(ITracer tracer, Connection connection, Action`3 handleRequest)
   at GVFS.Common.NamedPipes.NamedPipeServer.OnNewConnection(IAsyncResult ar, Boolean createNewThreadIfSynchronous)\",
"ErrorMessage":"Unhandled exception in connection handler"

Timeline of events

1. git rebase is started (and acquires the lock)
2. git rebase hangs
3. User runs gvfs diagnose
4. gvfs diagnose unmounts and remounts the repo
5. As the repo is unmounted, git rebase hits an error that causes the git command to exit
6. The post-command hook called by git rebase crosses with GVFS.Mount starting up, and GVFS.Mount receives a NamedPipeMessages.ReleaseLock.Request before InProcessMount.fileSystemCallbacks is initialized.

The crash occurs in HandleReleaseLockRequest because it does not check if fileSystemCallbacks is valid (nor does it check the if the repo is Ready like HandleLockRequest).

Start in GVFS.Mount/InProcessMount.cs at HandleReleaseLockRequest, then compare its readiness handling with HandleLockRequest. Reproduce or trace the startup race described in the timeline, and verify that a ReleaseLock received before fileSystemCallbacks is initialized no longer causes an unhandled NullReferenceException.