Hard crash on FocusLost

[nosami]

ERROR [2019-06-26 13:58:10Z]: An unhandled exception has occurred. Terminating Visual Studio? False
System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.Collections.Generic.KeyNotFoundException: The given key '/Users/jasonimison/Projects/newtonsoftpdbpackage/newtonsoftpdbpackage/Program.cs' was not present in the dictionary.
  at System.Collections.Generic.Dictionary`2[TKey,TValue].get_Item (TKey key) [0x0001e] in /Users/builder/jenkins/workspace/build-package-osx-mono/2019-02/external/bockbuild/builds/mono-x64/external/corefx/src/Common/src/CoreLib/System/Collections/Generic/Dictionary.cs:226
  at <StartupCode$XSVim>.$Addin+focusLost@136-4.Invoke (System.EventArgs _arg3) [0x00000] in <5cf5019b2429f754a74503839b01f55c>:0
  at Microsoft.FSharp.Control.CommonExtensions+SubscribeToObservable@1722[T].System-IObserver`1-OnNext (T args) [0x00000] in <5c64a986dff9fae1a745038386a9645c>:0
  at Microsoft.FSharp.Core.CompilerServices.RuntimeHelpers+h@349[TArgs].Invoke (System.Object _arg1, TArgs args) [0x00000] in <5c64a986dff9fae1a745038386a9645c>:0
  at <StartupCode$XSVim>.$Addin+focusLost@135-3.Invoke (System.Object delegateArg0, System.EventArgs delegateArg1) [0x00000] in <5cf5019b2429f754a74503839b01f55c>:0
  at (wrapper delegate-invoke) <Module>.invoke_void_object_EventArgs(object,System.EventArgs)
  at MonoDevelop.SourceEditor.SourceEditorView.TextArea_FocusOutEvent (System.Object o, Gtk.FocusOutEventArgs args) [0x00000] in /Users/vsts/agent/2.1

[nosami]

Just saw this error again while looking at a sourcelink file and then deleting the file on disk while the IDE was still running.
Probably just needs a simple check to see if the key exists and ignore if it doesn't