Two WSA plugins sharing same placeholder produces errors in 5.4

In Unity 5.4.0f3, I have a project where two plugins share the same placeholder and that produces an error that it never used to before Unity 5.4:

Code (CSharp):

1. GetFileByPath(filePath) == NULL
2. UnityEditor.HostView:OnGUI()

I had two plugins share the same placeholder because the plugin I wrote uses a third-party API that has different API for Windows 8.1 and Windows Phone 8.1. To make things easier, I built one version of the plugin for Windows 8.1, and set it in Unity to only compile for Windows 8.1 SDK, and the other version would be built for and set to only compile for Windows Phone 8.1 SDK. They both shared a stub placeholder DLL since the methods were the same and this setup worked pretty well. Now it causes that error. I can probably think of a workaround but I feel like this is a Unity bug.

 

Case 822480