Crash of the language server

[MikaelMayer]

Failing code

I cannot post minimal code.

Steps to reproduce the issue

• Dafny version: 3.8.1.40901
• Dafny VSCode extension version: latest master (local) dafny-lang/ide-vscode@f70b4f9

Expected behavior

No error should appear

Actual behavior

[Error - 11:58:32 AM] Request textDocument/documentSymbol failed.
  Message: Internal Error - System.InvalidOperationException: Sequence contains no elements
   at System.Linq.ThrowHelper.ThrowNoElementsException()
   at System.Linq.Enumerable.Last[TSource](IEnumerable`1 source)
   at Microsoft.Dafny.ArrowType.PrettyArrowTypeName(String arrow, List`1 typeArgs, Type result, ModuleDefinition context, Boolean parseAble)
   at Microsoft.Dafny.UserDefinedType.TypeName(ModuleDefinition context, Boolean parseAble)
   at Microsoft.Dafny.Type.<>c__DisplayClass21_0.<TypeArgsToString>b__0(Type ty)
   at Microsoft.Dafny.Util.<>c__DisplayClass3_0`1.<Comma>b__0(T element, Int32 index)
   at Microsoft.Dafny.Util.Comma[T](String comma, IEnumerable`1 l, Func`3 f)
   at Microsoft.Dafny.Util.Comma[T](IEnumerable`1 l, Func`2 f)
   at Microsoft.Dafny.Type.TypeArgsToString(ModuleDefinition context, List`1 typeArgs, Boolean parseAble)
   at Microsoft.Dafny.Type.TypeArgsToString(ModuleDefinition context, Boolean parseAble)
   at Microsoft.Dafny.MapType.TypeName(ModuleDefinition context, Boolean parseAble)
   at Microsoft.Dafny.Type.ToString()
   at Microsoft.Dafny.TypeConstraint.ErrorMsg.<Reporting>g__RemoveAmbiguity|5_0(Object[] msgArgs)
   at Microsoft.Dafny.TypeConstraint.ErrorMsg.Reporting(ErrorReporter reporter, String suffix)
   at Microsoft.Dafny.TypeConstraint.ErrorMsg.ReportAsError(ErrorReporter reporter)
   at Microsoft.Dafny.TypeConstraint.ReportErrors(ErrorReporter reporter)
   at Microsoft.Dafny.Resolver.SolveAllTypeConstraints()
   at Microsoft.Dafny.Resolver.ResolveTopLevelDecls_Core(List`1 declarations, Graph`1 datatypeDependencies, Graph`1 codatatypeDependencies, Boolean isAnExport)
   at Microsoft.Dafny.Resolver.ResolveProgram(Program prog)
   at Microsoft.Dafny.LanguageServer.Language.Symbols.DafnyLangSymbolResolver.RunDafnyResolver(TextDocumentItem document, Program program) in `dafny\Source\DafnyLanguageServer\Language\Symbols\DafnyLangSymbolResolver.cs:line 49
   at Microsoft.Dafny.LanguageServer.Language.Symbols.DafnyLangSymbolResolver.ResolveSymbols(TextDocumentItem textDocument, Program program, Boolean& canDoVerification, CancellationToken cancellationToken) in `dafny\Source\DafnyLanguageServer\Language\Symbols\DafnyLangSymbolResolver.cs:line 31
   at Microsoft.Dafny.LanguageServer.Workspace.TextDocumentLoader.LoadInternal(DocumentTextBuffer textDocument, CancellationToken cancellationToken) in `dafny\Source\DafnyLanguageServer\Workspace\TextDocumentLoader.cs:line 95
   at Microsoft.Dafny.LanguageServer.Workspace.TextDocumentLoader.<>c__DisplayClass12_0.<<LoadAsync>b__0>d.MoveNext() in `dafny\Source\DafnyLanguageServer\Workspace\TextDocumentLoader.cs:line 78
--- End of stack trace from previous location ---
   at Microsoft.Dafny.LanguageServer.Workspace.TextDocumentLoader.LoadAsync(DocumentTextBuffer textDocument, CancellationToken cancellationToken) in `dafny\Source\DafnyLanguageServer\Workspace\TextDocumentLoader.cs:line 77
   at Microsoft.Dafny.LanguageServer.Workspace.Compilation.ResolveAsync() in `dafny\Source\DafnyLanguageServer\Workspace\Compilation.cs:line 85
   at Microsoft.Dafny.LanguageServer.Workspace.DocumentManager.GetSnapshotAfterResolutionAsync() in `dafny\Source\DafnyLanguageServer\Workspace\DocumentManager.cs:line 177
   at Microsoft.Dafny.LanguageServer.Handlers.DafnyDocumentSymbolHandler.Handle(DocumentSymbolParams request, CancellationToken cancellationToken) in `dafny\Source\DafnyLanguageServer\Handlers\DafnyDocumentSymbolHandler.cs:line 34
   at OmniSharp.Extensions.LanguageServer.Server.Pipelines.SemanticTokensDeltaPipeline`2.Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate`1 next)
   at OmniSharp.Extensions.LanguageServer.Server.Pipelines.ResolveCommandPipeline`2.Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate`1 next)
   at MediatR.Pipeline.RequestPreProcessorBehavior`2.Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate`1 next)
   at MediatR.Pipeline.RequestPostProcessorBehavior`2.Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate`1 next)
   at MediatR.Pipeline.RequestExceptionProcessorBehavior`2.Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate`1 next)
   at MediatR.Pipeline.RequestExceptionProcessorBehavior`2.Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate`1 next)
   at MediatR.Pipeline.RequestExceptionActionProcessorBehavior`2.Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate`1 next)
   at MediatR.Pipeline.RequestExceptionActionProcessorBehavior`2.Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate`1 next)
   at OmniSharp.Extensions.JsonRpc.RequestRouterBase`1.<RouteRequest>g__InnerRoute|7_0(IServiceScopeFactory serviceScopeFactory, Request request, TDescriptor descriptor, Object params, CancellationToken token, ILogger logger)
   at OmniSharp.Extensions.JsonRpc.RequestRouterBase`1.RouteRequest(IRequestDescriptor`1 descriptors, Request request, CancellationToken token)
   at OmniSharp.Extensions.JsonRpc.DefaultRequestInvoker.<>c__Displ