Fix #2545: LINQ expression has a question operator on the source collection

ICSharpCode.Decompiler.Tests/TestCases/Pretty/QueryExpressions.cs

@@ -204,6 +204,13 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 				   select (x);
 		}
 
+#if CS60
+		private List<string> Issue2545(List<string> arglist)
+		{
+			return arglist?.OrderByDescending((string f) => f.Length).ThenBy((string f) => f.ToLower()).ToList();
+		}
+#endif
+
 		public static IEnumerable<char> Issue1310a(bool test)
 		{
 #if ROSLYN && OPT

ICSharpCode.Decompiler/CSharp/Transforms/IntroduceQueryExpressions.cs

@@ -361,7 +361,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				return false;
 
 			if (mre.MemberName == "OrderBy" || mre.MemberName == "OrderByDescending")
-				return true;
+				return !IsNullConditional(mre.Target);
 			else if (mre.MemberName == "ThenBy" || mre.MemberName == "ThenByDescending")
 				return ValidateThenByChain(mre.Target as InvocationExpression, expectedParameterName);
 			else