fixed bug in identifier

Author: david-driscoll

src/Protocol/Models/TextDocumentIdentifier.cs

@@ -42,7 +42,7 @@ public override bool Equals(object? obj)
 
         public static implicit operator TextDocumentIdentifier(string uri) => new TextDocumentIdentifier { Uri = uri };
 
-        private string DebuggerDisplay => Uri.ToString()!;
+        private string DebuggerDisplay => ((Uri?)Uri)?.ToString() ?? string.Empty;
 
         /// <inheritdoc />
         public override string ToString() => DebuggerDisplay;