[LSP Snippets] Performs null check for _cap.CompletionItem.

src/OmniSharp.LanguageServerProtocol/Handlers/CompletionHandler.cs

@@ -81,7 +81,7 @@ public async Task<CompletionList> Handle(CompletionParams request, CancellationT
                 WantKind = true,
                 WantDocumentationForEveryCompletionResult = true,
                 WantReturnType = true,
-                WantSnippet =_capability.CompletionItem.SnippetSupport,
+                WantSnippet =_capability.CompletionItem?.SnippetSupport ?? false
             };
 
             var omnisharpResponse = await _autoCompleteHandler.Handle(omnisharpRequest);