OmniSharpDocumentSymbolHandler.cs: a fix for a crash in 'textDocument…

…/documentSymbol'

The NRE may happen when OS.R.CS.Services.SStructre.CodeStructureService
returns a null during the time when projects are still being loaded but
LSP client already sends queries for symbol info under cursor.

Related to OmniSharp#2141

src/OmniSharp.LanguageServerProtocol/Handlers/OmniSharpDocumentSymbolHandler.cs

@@ -41,7 +41,7 @@ public override async Task<SymbolInformationOrDocumentSymbolContainer> Handle(Do
 
             var omnisharpResponse = await _codeStructureHandler.Handle(omnisharpRequest);
 
-            return omnisharpResponse.Elements?.Select(ToDocumentSymbolInformationOrDocumentSymbol).ToArray() ??
+            return omnisharpResponse?.Elements?.Select(ToDocumentSymbolInformationOrDocumentSymbol).ToArray() ??
                 Array.Empty<SymbolInformationOrDocumentSymbol>();
         }