Fix #2230: do not emit directly consecutive null propagation operators.

ICSharpCode.Decompiler/IL/Transforms/NullPropagationTransform.cs

@@ -305,6 +305,11 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 					{
 						inst = addressOf.Value;
 					}
+					// The argument of the unwrap instruction cannot be the end of the chain, because this
+					// would result in two null-conditional operators immediately following each other,
+					// which is not valid in C#.
+					if (IsValidEndOfChain())
+						return false;
 				}
 				else if (inst is DynamicGetMemberInstruction dynGetMember)
 				{