Collection was modified error

[mrward]

The following error was logged. I suspect that a language client document was closed at the same time as the solution was closed, and the session was removed when the document closed whilst the sessions were all being closed. The code should convert the enumerable to an array and also check if it still in the dictionary to catch it the case where it has already been removed.

Async operation failed
System.AggregateException: One or more errors occurred. ---> System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
  at System.ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumFailedVersion () [0x00000] in /Users/builder/jenkins/workspace/build-package-osx-mono/2017-12/external/bockbuild/builds/mono-x64/mcs/class/referencesource/mscorlib/system/throwhelper.cs:130 
  at System.Collections.Generic.Dictionary`2+ValueCollection+Enumerator[TKey,TValue].MoveNext () [0x00013] in /Users/builder/jenkins/workspace/build-package-osx-mono/2017-12/external/bockbuild/builds/mono-x64/external/corefx/src/Common/src/CoreLib/System/Collections/Generic/Dictionary.cs:1468 
  at MonoDevelop.LanguageServer.Client.SolutionLanguageClientSessions+<GetAllSessions>d__7.MoveNext () [0x000bd] in monodevelop-language-server-addin/src/MonoDevelop.LanguageServer.Client/MonoDevelop.LanguageServer.Client/SolutionLanguageClientSessions.cs:95 
  at MonoDevelop.LanguageServer.Client.LanguageClientWorkspace+<ShutdownAllSessions>d__21.MoveNext () [0x000dc] in monodevelop-language-server-addin/src/MonoDevelop.LanguageServer.Client/MonoDevelop.LanguageServer.Client/LanguageClientWorkspace.cs:230 
--- End of stack trace from previous location where exception was thrown ---
  at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x0000c] in /Users/builder/jenkins/workspace/build-package-osx-mono/2017-12/external/bockbuild/builds/mono-x64/mcs/class/referencesource/mscorlib/system/runtime/exceptionservices/exceptionservicescommon.cs:152 
  at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (System.Threading.Tasks.Task task) [0x00037] in /Users/builder/jenkins/workspace/build-package-osx-mono/2017-12/external/bockbuild/builds/mono-x64/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:187 
  at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (System.Threading.Tasks.Task task) [0x00028] in /Users/builder/jenkins/workspace/build-package-osx-mono/2017-12/external/bockbuild/builds/mono-x64/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:156 
  at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd (System.Threading.Tasks.Task task) [0x00008] in /Users/builder/jenkins/workspace/build-package-osx-mono/2017-12/external/bockbuild/builds/mono-x64/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:128 
  at System.Runtime.CompilerServices.TaskAwaiter.GetResult () [0x00000] in /Users/builder/jenkins/workspace/build-package-osx-mono/2017-12/external/bockbuild/builds/mono-x64/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:113 
  at MonoDevelop.LanguageServer.Client.LanguageClientWorkspace+<ShutdownAllSessions>d__20.MoveNext () [0x00030] in monodevelop-language-server-addin/src/MonoDevelop.LanguageServer.Client/MonoDevelop.LanguageServer.Client/LanguageClientWorkspace.cs:224 
   --- End of inner exception stack trace ---
---> (Inner Exception #0) System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
  at System.ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumFailedVersion () [0x00000] in /Users/builder/jenkins/workspace/build-package-osx-mono/2017-12/external/bockbuild/builds/mono-x64/mcs/class/referencesource/mscorlib/system/throwhelper.cs:130 
  at System.Collections.Generic.Dictionary`2+ValueCollection+Enumerator[TKey,TValue].MoveNext () [0x00013] in /Users/builder/jenkins/workspace/build-package-osx-mono/2017-12/external/bockbuild/builds/mono-x64/external/corefx/src/Common/src/CoreLib/System/Collections/Generic/Dictionary.cs:1468 
  at MonoDevelop.LanguageServer.Client.SolutionLanguageClientSessions+<GetAllSessions>d__7.MoveNext () [0x000bd] in monodevelop-language-server-addin/src/MonoDevelop.LanguageServer.Client/MonoDevelop.LanguageServer.Client/SolutionLanguageClientSessions.cs:95 
  at MonoDevelop.LanguageServer.Client.LanguageClientWorkspace+<ShutdownAllSessions>d__21.MoveNext () [0x000dc] in monodevelop-language-server-addin/src/MonoDevelop.LanguageServer.Client/MonoDevelop.LanguageServer.Client/LanguageClientWorkspace.cs:230 
--- End of stack trace from previous location where exception was thrown ---
  at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x0000c] in /Users/builder/jenkins/workspace/build-package-osx-mono/2017-12/external/bockbuild/builds/mono-x64/mcs/class/referencesource/mscorlib/system/runtime/exceptionservices/exceptionservicescommon.cs:152 
  at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (System.Threading.Tasks.Task task) [0x00037] in /Users/builder/jenkins/workspace/build-package-osx-mono/2017-12/external/bockbuild/builds/mono-x64/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:187 
  at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (System.Threading.Tasks.Task task) [0x00028] in /Users/builder/jenkins/workspace/build-package-osx-mono/2017-12/external/bockbuild/builds/mono-x64/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:156 
  at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd (System.Threading.Tasks.Task task) [0x00008] in /Users/builder/jenkins/workspace/build-package-osx-mono/2017-12/external/bockbuild/builds/mono-x64/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:128 
  at System.Runtime.CompilerServices.TaskAwaiter.GetResult () [0x00000] in /Users/builder/jenkins/workspace/build-package-osx-mono/2017-12/external/bockbuild/builds/mono-x64/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:113 
  at MonoDevelop.LanguageServer.Client.LanguageClientWorkspace+<ShutdownAllSessions>d__20.MoveNext () [0x00030] in monodevelop-language-server-addin/src/MonoDevelop.LanguageServer.Client/MonoDevelop.LanguageServer.Client/LanguageClientWorkspace.cs:224 <---