[banshee] MediaEngineTests: add a sanity check to AssertTransition()

[Andrés Aragoneses <aaragoneses src gnome org>]

commit 201efbeca06cfac8df5ab0cc2595a3cc3ce97940
Author: Andrés G. Aragoneses <knocte gmail com>
Date:   Fri Jan 31 00:14:07 2014 +0100

    MediaEngineTests: add a sanity check to AssertTransition()
    
    If this sanity check had been there before, I wouldn't have had
    scratched my head for so long! This happens when using dynamic-ish
    things like the 'object' type...

 .../Banshee.Services/Banshee.MediaEngine/Tests.cs  |   10 ++++++++++
 1 files changed, 10 insertions(+), 0 deletions(-)
---
diff --git a/src/Core/Banshee.Services/Banshee.MediaEngine/Tests.cs 
b/src/Core/Banshee.Services/Banshee.MediaEngine/Tests.cs
index 97dd0d2..60ab3e4 100644
--- a/src/Core/Banshee.Services/Banshee.MediaEngine/Tests.cs
+++ b/src/Core/Banshee.Services/Banshee.MediaEngine/Tests.cs
@@ -200,6 +200,16 @@ namespace Banshee.MediaEngine
         private void AssertTransition (System.Func<PlayerState?, PlayerEvent?, bool> ignore, System.Action 
action, params object [] states)
         {
             Log.DebugFormat ("AssertTransition: {0}", String.Join (", ", states.Select (s => s.ToString 
()).ToArray ()));
+
+            foreach (var state in states) {
+                if (state == null) {
+                    throw new ArgumentException ("states array cannot contain nulls");
+                }
+                if (state.GetType () != typeof (PlayerEvent) && state.GetType () != typeof (PlayerState)) {
+                    throw new ArgumentException ("states array can only contain states or events, not " + 
state.GetType ().FullName);
+                }
+            }
+
             int result_count = 0;
             var reset_event = new ManualResetEvent (false);
             var handler = new Action<PlayerEventArgs> (a => {