Tree Problems {Please Help}

So i'm making a horror game. horror games usually have trees. i'm using bigtrees and when i add a mesh collider to stop the player from being able to walk through them i get this error message
Failed getting triangles. Submesh 2 has no indices. Mesh asset path "Assets/Standard Assets/Tree Creator/Trees/BigTree.prefab" Mesh name "Mesh"
UnityEngine.GUIUtilityrocessEvent(Int32, IntPtr)
If i need to put anything else here please reply