Crash on autosync if fuse-sshfs missing

[samtygier]

On Fedora 27, If I have sync via ssh enabled, but don't have fuse-sshfs installed I get the following exception:

Unhandled Exception:
System.NullReferenceException: Object reference not set to an instance of an object
  at Tomboy.Sync.FuseSyncServiceAddin.PostSyncCleanup () [0x00001] in <e9b7244e2afe4ca6b53858505fa3d1ce>:0 
  at Tomboy.Sync.SyncManager.BackgroundSyncChecker () [0x00076] in <e9b7244e2afe4ca6b53858505fa3d1ce>:0 
  at Tomboy.Sync.SyncManager.<UpdateSyncAction>m__4 (System.Object o) [0x00000] in <e9b7244e2afe4ca6b53858505fa3d1ce>:0 
  at System.Threading.Timer+Scheduler.TimerCB (System.Object o) [0x00007] in <373b6e083d6e45e498c9082a8eebd27f>:0 
  at System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem () [0x00019] in <373b6e083d6e45e498c9082a8eebd27f>:0 
  at System.Threading.ThreadPoolWorkQueue.Dispatch () [0x00096] in <373b6e083d6e45e498c9082a8eebd27f>:0 
  at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback () [0x00000] in <373b6e083d6e45e498c9082a8eebd27f>:0 
[ERROR] FATAL UNHANDLED EXCEPTION: System.NullReferenceException: Object reference not set to an instance of an object
  at Tomboy.Sync.FuseSyncServiceAddin.PostSyncCleanup () [0x00001] in <e9b7244e2afe4ca6b53858505fa3d1ce>:0 
  at Tomboy.Sync.SyncManager.BackgroundSyncChecker () [0x00076] in <e9b7244e2afe4ca6b53858505fa3d1ce>:0 
  at Tomboy.Sync.SyncManager.<UpdateSyncAction>m__4 (System.Object o) [0x00000] in <e9b7244e2afe4ca6b53858505fa3d1ce>:0 
  at System.Threading.Timer+Scheduler.TimerCB (System.Object o) [0x00007] in <373b6e083d6e45e498c9082a8eebd27f>:0 
  at System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem () [0x00019] in <373b6e083d6e45e498c9082a8eebd27f>:0 
  at System.Threading.ThreadPoolWorkQueue.Dispatch () [0x00096] in <373b6e083d6e45e498c9082a8eebd27f>:0 
  at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback () [0x00000] in <373b6e083d6e45e498c9082a8eebd27f>:0

This is with Tomboy 1.15.9.

Fedora bug report: https://bugzilla.redhat.com/show_bug.cgi?id=1411854