add device null check

MediaBrowser.Server.Implementations/Session/SessionManager.cs

@@ -432,10 +432,13 @@ namespace MediaBrowser.Server.Implementations.Session
 
                 device = device ?? _deviceManager.GetDevice(deviceId);
 
+                if (device != null)
+                {
                     if (!string.IsNullOrEmpty(device.CustomName))
                     {
                         deviceName = device.CustomName;
                     }
+                }
 
                 sessionInfo.DeviceName = deviceName;
                 sessionInfo.UserId = userId;