Fix nullability warnings in AstNode and XmlDocumentationElement.

ICSharpCode.Decompiler/CSharp/Syntax/AstNode.cs

@@ -358,9 +358,9 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			return Ancestors.OfType<T>().FirstOrDefault();
 		}
 
-		public AstNode GetParent(Func<AstNode, bool>? pred)
+		public AstNode? GetParent(Func<AstNode, bool>? pred)
 		{
-			return Ancestors.FirstOrDefault(pred);
+			return pred != null ? Ancestors.FirstOrDefault(pred) : Ancestors.FirstOrDefault();
 		}
 
 		public AstNodeCollection<T> GetChildrenByRole<T>(Role<T> role) where T : AstNode

ICSharpCode.Decompiler/Documentation/XmlDocumentationElement.cs

@@ -118,7 +118,7 @@ namespace ICSharpCode.Decompiler.Documentation
 		/// </summary>
 		public string? GetAttribute(string? name)
 		{
-			return element?.Attribute(name)?.Value;
+			return name == null ? null : element?.Attribute(name)?.Value;
 		}
 
 		/// <summary>
@@ -212,7 +212,7 @@ namespace ICSharpCode.Decompiler.Documentation
 							var doc = XDocument.Parse(inheritedDocumentation).Element("doc");
 
 							// XPath filter not yet implemented
-							if (childElement.Parent?.Parent == null && childElement.Attribute("select")?.Value == null)
+							if (doc != null && childElement.Parent?.Parent == null && childElement.Attribute("select")?.Value == null)
 							{
 								// Inheriting documentation at the root level
 								List<string> doNotInherit = new List<string>();