Migo: NRE on dispose

[arfbtwn]

After Banshee closes we sometimes see this stack trace:

Unhandled Exception:
System.NullReferenceException: Object reference not set to an instance of an object
  at Migo.TaskCore.TaskGroup`1[T].PumpQueue () [0x00015] in /tmp/portage/media-sound/banshee-9999/work/banshee-9999/src/Libraries/Migo/Migo.TaskCore/TaskGroup.cs:766 
  at System.Threading.ThreadHelper.ThreadStart_Context (System.Object state) [0x00014] in /var/tmp/portage/dev-lang/mono-5.2.0.196/work/mono-5.2.0.196-abi_x86_64.amd64/mcs/class/referencesource/mscorlib/system/threading/thread.cs:68 
  at System.Threading.ExecutionContext.RunInternal (System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state, System.Boolean preserveSyncCtx) [0x00071] in /var/tmp/portage/dev-lang/mono-5.2.0.196/work/mono-5.2.0.196-abi_x86_64.amd64/mcs/class/referencesource/mscorlib/system/threading/executioncontext.cs:957 
  at System.Threading.ExecutionContext.Run (System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state, System.Boolean preserveSyncCtx) [0x00000] in /var/tmp/portage/dev-lang/mono-5.2.0.196/work/mono-5.2.0.196-abi_x86_64.amd64/mcs/class/referencesource/mscorlib/system/threading/executioncontext.cs:904 
  at System.Threading.ExecutionContext.Run (System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state) [0x0002b] in /var/tmp/portage/dev-lang/mono-5.2.0.196/work/mono-5.2.0.196-abi_x86_64.amd64/mcs/class/referencesource/mscorlib/system/threading/executioncontext.cs:893 
  at System.Threading.ThreadHelper.ThreadStart () [0x00008] in /var/tmp/portage/dev-lang/mono-5.2.0.196/work/mono-5.2.0.196-abi_x86_64.amd64/mcs/class/referencesource/mscorlib/system/threading/thread.cs:105 
[ERROR] FATAL UNHANDLED EXCEPTION: System.NullReferenceException: Object reference not set to an instance of an object
  at Migo.TaskCore.TaskGroup`1[T].PumpQueue () [0x00015] in /tmp/portage/media-sound/banshee-9999/work/banshee-9999/src/Libraries/Migo/Migo.TaskCore/TaskGroup.cs:766 
  at System.Threading.ThreadHelper.ThreadStart_Context (System.Object state) [0x00014] in /var/tmp/portage/dev-lang/mono-5.2.0.196/work/mono-5.2.0.196-abi_x86_64.amd64/mcs/class/referencesource/mscorlib/system/threading/thread.cs:68 
  at System.Threading.ExecutionContext.RunInternal (System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state, System.Boolean preserveSyncCtx) [0x00071] in /var/tmp/portage/dev-lang/mono-5.2.0.196/work/mono-5.2.0.196-abi_x86_64.amd64/mcs/class/referencesource/mscorlib/system/threading/executioncontext.cs:957 
  at System.Threading.ExecutionContext.Run (System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state, System.Boolean preserveSyncCtx) [0x00000] in /var/tmp/portage/dev-lang/mono-5.2.0.196/work/mono-5.2.0.196-abi_x86_64.amd64/mcs/class/referencesource/mscorlib/system/threading/executioncontext.cs:904 
  at System.Threading.ExecutionContext.Run (System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state) [0x0002b] in /var/tmp/portage/dev-lang/mono-5.2.0.196/work/mono-5.2.0.196-abi_x86_64.amd64/mcs/class/referencesource/mscorlib/system/threading/executioncontext.cs:893 
  at System.Threading.ThreadHelper.ThreadStart () [0x00008] in /var/tmp/portage/dev-lang/mono-5.2.0.196/work/mono-5.2.0.196-abi_x86_64.amd64/mcs/class/referencesource/mscorlib/system/threading/thread.cs:105