I get the following error when an update is needed (via buttons in the Package Manager):

PrjCommandException: Error occurred during 'Updating solution': Error: Could not find unity-generated project when generating custom solution. This is necessary so the custom solution can add things like unity defines and DLL references within the unity project.

Projeny.Internal.PrjCommandHandler+d__3.MoveNext ()
Projeny.Internal.CoRoutine.Pump ()
Rethrow as CoRoutineException: Coroutine Object Trace: PmVsSolutionViewHandler -> PrjCommandHandler
Projeny.Internal.CoRoutine.Pump ()
Projeny.Internal.AsyncProcessor.AdvanceFrameAll ()
Rethrow as AsyncOperationException: Error occurred during async operation
Projeny.Internal.AsyncProcessor.AdvanceFrameAll ()
Projeny.Internal.AsyncProcessor.Tick ()
Projeny.Internal.PmCompositionRoot.Update ()
Projeny.Internal.PmWindow.Update ()
UnityEngine.Debug:LogException(Exception)
Projeny.Internal.Log:ErrorException(Exception)
Projeny.Internal.PmWindow:OnErrorOccurred(Exception)
Projeny.Internal.PmWindow:Update()
UnityEditor.EditorApplication:Internal_CallUpdateFunctions()