Repro steps:
- make a new project in 2019.4 - latest version
- open Package Manager
- click to select the Proejct Manifest - but kep the Console windo in focus
- select a pacakge that needs updating
- update the package
- after updating the package, you will see this eror in the console:

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.PackageManager.PackageManifestImporterEditor.OnInspectorGUI () (at <06df0f95306b448dbd8fddaa73427228>:0)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass58_0.<CreateIMGUIInspectorFromEditor>b__0 () (at <06df0f95306b448dbd8fddaa73427228>:0)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

Reproducible in 2019.4 - latest, and tested all the way back to 2019.4.24f1

Cannot repro in any versions newer than 2019.4, so this is a bug for 2019.4 ONLY