Likely to be cause by incorrect pathing in package.json file, where forward slash is used instead of double back slask. In some environments, the forward slash does not result in any errors and is working as intended. The issue is also no longer reproducible if we move the package from package cache and import it to a project with the "Add package from disk.."

To reproduce:
1. In empty project, import Mobile Notifications package. Close and open the package manager again to see the window being empty
2. Observe the console

Expected: no errors are thrown
Actual: FileNotFoundExeption is thrown due to pathing issues

Reproduced in: 1.0.4 - preview.2, 1.2.1 - preview
Not reproduced in: 1.0.4 - preview.1

Note1: issue is reproducible from the release that has Notification Samples added
Note2: full error:

FileNotFoundException: Could not find file 'G:\GithubProjects\com.unity.mobile.notifications\TestProjects\NotificationSamples_2019_3\Library\PackageCache\com.unity.mobile.notifications@1.2.1-preview\Samples~/NotificationsSamples\TextMesh Pro\Resources\Fonts & Materials\LiberationSans SDF - Drop Shadow.mat'.
System.IO.__Error.WinIOError (System.Int32 errorCode, System.String maybeFullPath) (at <437ba245d8404784b9fbab9b439ac908>:0)
System.IO.FileInfo.get_Length () (at <437ba245d8404784b9fbab9b439ac908>:0)
(wrapper remoting-invoke-with-check) System.IO.FileInfo.get_Length()
UnityEditor.PackageManager.UI.IOUtils.DirectorySizeInBytes (System.String path) (at <a20a50bbfaff4193bf6ba6b262f9a71f>:0)
UnityEditor.PackageManager.UI.Sample.get_size () (at <a20a50bbfaff4193bf6ba6b262f9a71f>:0)
UnityEditor.PackageManager.UI.PackageSampleItem..ctor (UnityEditor.PackageManager.UI.IPackageVersion version, UnityEditor.PackageManager.UI.Sample sample) (at <a20a50bbfaff4193bf6ba6b262f9a71f>:0)
UnityEditor.PackageManager.UI.PackageSampleList.SetPackageVersion (UnityEditor.PackageManager.UI.IPackageVersion version) (at <a20a50bbfaff4193bf6ba6b262f9a71f>:0)
UnityEditor.PackageManager.UI.PackageDetails.SetDisplayVersion (UnityEditor.PackageManager.UI.IPackageVersion version) (at <a20a50bbfaff4193bf6ba6b262f9a71f>:0)
UnityEditor.PackageManager.UI.PackageDetails.SetPackage (UnityEditor.PackageManager.UI.IPackage package, UnityEditor.PackageManager.UI.IPackageVersion version) (at <a20a50bbfaff4193bf6ba6b262f9a71f>:0)
UnityEditor.PackageManager.UI.PackageDetails.OnSelectionChanged (UnityEditor.PackageManager.UI.IPackageVersion version) (at <a20a50bbfaff4193bf6ba6b262f9a71f>:0)
UnityEditor.PackageManager.UI.PackageDetails.OnEnable () (at <a20a50bbfaff4193bf6ba6b262f9a71f>:0)
UnityEditor.PackageManager.UI.PackageManagerWindow.OnEnable () (at <a20a50bbfaff4193bf6ba6b262f9a71f>:0)
UnityEditor.PackageManager.UI.Window:ShowPackageManagerWindow(MenuCommand)