I'm getting loads of error messages that say

Material doesn't have a texture property '_MainTex'
UnityEditor.DockArea:OnGUI()

I dug into the Editor logs and the precise message is

[/Applications/buildAgent/work/6bc5f79e0a4296d6/Runtime/Shaders/Material.cpp line 562] 
(Filename: /Applications/buildAgent/work/6bc5f79e0a4296d6/Runtime/Shaders/Material.cpp Line: 562)

Material doesn't have a texture property '_MainTex'
UnityEngine.DetailRenderer:Render(Camera, Single, Int32, Single)
UnityEngine.Terrain:CullAllTerrains(Int32)
UnityEditor.Handles:Internal_DrawCameraWithGrid(Camera, Int32, DrawGridParameters&)
UnityEditor.Handles:DrawCameraImpl(Rect, Camera, DrawCameraMode, Boolean, DrawGridParameters, Boolean)
UnityEditor.Handles:DrawCameraStep1(Rect, Camera, DrawCameraMode, DrawGridParameters)
UnityEditor.SceneView:OnGUI()
System.Reflection.MonoMethod:InternalInvoke(Object, Object[], Exception&)
System.Reflection.MonoMethod:Invoke(Object, BindingFlags, Binder, Object[], CultureInfo)
System.Reflection.MethodBase:Invoke(Object, Object[])
UnityEditor.HostView:Invoke(String, Object)
UnityEditor.HostView:Invoke(String)
UnityEditor.DockArea:OnGUI()

They stop if i delete the terrain, but persist even with a new terrain in a new scene. I'm sure I could work around this, but I'm curious now and want to track down why this is breaking. I have updated the project from pre-Unity3 to the current version, but wasn't getting this error before. Alternatively, does anyone have a way to hone in on materials/prefabs/assets that are causing problems within a project without moving or disrupting the whole project? Can you selectively disable them or something?

Thanks for your help

dylan