Do you mind sharing the project you are using with 2018.4 (or any other project that repro the issue) that include quicksearch in the manifest.json? I've created a new project with 2018.4.4f1 and I didn't get that issue. Here's the zip of the project. Can you try to open it and see if you still get the same error? ExCSS.Unity is related to UIElements USS files so I wonder if you have another plugin/package that could mess up with the *.uss files.