Check for null sessions and requests in SyncPlay

MediaBrowser.Model/SyncPlay/GroupRequestType.cs

@@ -0,0 +1,33 @@
+namespace MediaBrowser.Model.SyncPlay
+{
+    /// <summary>
+    /// Enum GroupRequestType.
+    /// </summary>
+    public enum GroupRequestType
+    {
+        /// <summary>
+        /// A user is requesting to create a new group.
+        /// </summary>
+        NewGroup,
+
+        /// <summary>
+        /// A user is requesting to join a group.
+        /// </summary>
+        JoinGroup,
+
+        /// <summary>
+        /// A user is requesting to leave a group.
+        /// </summary>
+        LeaveGroup,
+
+        /// <summary>
+        /// A user is requesting the list of available groups.
+        /// </summary>
+        ListGroups,
+
+        /// <summary>
+        /// A user is sending a playback command to a group.
+        /// </summary>
+        Playback
+    }
+}