XmlDocumentationElement: return null if given a null entity

src/Libraries/NRefactory/ICSharpCode.NRefactory.Xml/DocumentationElement.cs

@@ -39,6 +39,8 @@ namespace ICSharpCode.NRefactory.Xml
 		/// </summary>
 		public static XmlDocumentationElement Get(IEntity entity, bool inheritDocIfMissing = true)
 		{
+			if (entity == null)
+				return null;
 			var documentationComment = entity.Documentation;
 			if (documentationComment != null) {
 				return Create(documentationComment, entity);