Some unit tests are simply broken and create an API with a null m_item.

This isn't supposed to be done and should be changed in the tests.
For now, add a protective check.

OpenSim/Region/ScriptEngine/Shared/Api/Implementation/LSL_Api.cs

@@ -403,6 +403,12 @@ namespace OpenSim.Region.ScriptEngine.Shared.Api
 
         protected virtual void Sleep(int delay)
         {
+            if (m_item == null) // Some unit tests don't set this
+            {
+                Thread.Sleep(delay);
+                return;
+            }
+
             m_ScriptEngine.SleepScript(m_item.ItemID, delay);
         }