public sealed class UnknownDatastore : Datastore, System.IDisposable  

public async Task ObtainingGeodatabaseFromProjectItem()
{
    IEnumerable<GDBProjectItem> gdbProjectItems = Project.Current.GetItems<GDBProjectItem>();

    await ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
    {
        foreach (GDBProjectItem gdbProjectItem in gdbProjectItems)
        {
            using (Datastore datastore = gdbProjectItem.GetDatastore())
            {
                //Unsupported datastores (non File GDB and non Enterprise GDB) will be of type UnknownDatastore
                if (datastore is UnknownDatastore)
                    continue;

                Geodatabase geodatabase = datastore as Geodatabase;
                // Use the geodatabase.
            }
        }
    });
}