Fail with null value instead of other exception in case of invalid json file.

ICSharpCode.Decompiler/DotNetCore/DotNetCorePathFinder.cs

@@ -93,7 +93,7 @@ namespace ICSharpCode.Decompiler
 			foreach (var library in libraries) {
 				var type = library.Value["type"].AsString;
 				var path = library.Value["path"].AsString;
-				var runtimeInfo = runtimeInfos[library.Key]["runtime"].AsJsonObject;
+				var runtimeInfo = runtimeInfos[library.Key].AsJsonObject?["runtime"].AsJsonObject;
 				string[] components = new string[runtimeInfo?.Count ?? 0];
 				if (runtimeInfo != null) {
 					int i = 0;