Any way to stop texture lookup wiping material from prefab?

I am currently trying to create a prefab based inventory system.

The objects in the inventory GUI will be displayed as the material.maintexture of the prefab in that slot, so if Slot 1 contains object A, the script should look up the prefab that is in Slot 1 (ObjectA) and set the normal.background of the GUI button to the material.maintexture of that prefab.

The code I am using is:

 public var m_PlayerObject : GameObject;
 public var m_PlayerScriptClone : PlayerScriptJava;
     m_PlayerScriptClone = m_PlayerObject.GetComponent(PlayerScriptJava) as PlayerScriptJava; //assigns clone of playerscript, where the inventory slots are located
 public var m_SlotA : GUIStyle;
 
 m_SlotA.normal.background = m_PlayerScriptClone.m_SlotAPrefab.renderer.material.mainTexture as Texture2D;

This works, but when I stop the game playing, it will 'cleanup' the material off the prefab assigned to m_SlotAPrefab. So when I play the game again, all materials which were previously looked up have now been taken off the prefabs. Is there any way to stop this?