BulletSim: remove some errors on shutdown by moving terrain destruction until after physical object destruction. TerrainManager also made disposable and that feature used.

OpenSim/Region/Physics/BulletSPlugin/BSScene.cs

@@ -349,8 +349,6 @@ public sealed class BSScene : PhysicsScene, IPhysicsParameters
         // make sure no stepping happens while we're deleting stuff
         m_initialized = false;
 
-        TerrainManager.ReleaseGroundPlaneAndTerrain();
-
         foreach (KeyValuePair<uint, BSPhysObject> kvp in PhysObjects)
         {
             kvp.Value.Destroy();
@@ -370,6 +368,13 @@ public sealed class BSScene : PhysicsScene, IPhysicsParameters
             Shapes = null;
         }
 
+        if (TerrainManager != null)
+        {
+            TerrainManager.ReleaseGroundPlaneAndTerrain();
+            TerrainManager.Dispose();
+            TerrainManager = null;
+        }
+
         // Anything left in the unmanaged code should be cleaned out
         BulletSimAPI.Shutdown2(World.ptr);