Error is thrown on clicking any .md file after clicking package.json.

Steps to repro:
1. Create a new project
2. Project window > Packages > Package Manager UI.
3. Click on Package.json
4. Click on any .md file available. (README, LICENSE, CHANGELOG, QAReport)

Actual Result:
"PackageManifestImporterEditor.OnEnable must call base.OnEnable to avoid unexpected behaviour.
UnityEditor.Experimental.AssetImporters.AssetImporterEditor:OnDisable()" error is thrown.

Expected Result:
There are no errors in the console.

Reproducible on:
2019.2.0a8 (18a4512f903f), 2019.2.0a6

Works fine on:
2019.1.0b7, 2019.2.0a5, 2018.3.0f2

Environment:
Windows and MAC