Custom inspector: function for assets?

An object field:

http://docs.unity3d.com/Documentation/ScriptReference/EditorGUILayout.ObjectField.html

I am using this for a sprite and whenever I try to assign it it gives me this error.

NullReferenceException: Object reference not set to an instance of an object UnityEditor.EditorGUI.ValidateObjectFieldAssignment (UnityEngine.Object[] references, System.Type objType, UnityEditor.SerializedProperty property) (at C:/BuildAgent/work/d3d49558e4d408f4/artifacts/EditorGenerated/EditorGUI.cs:2454) UnityEditor.EditorGUI.DoObjectField (Rect position, Rect dropRect, Int32 id, UnityEngine.Object obj, System.Type objType, UnityEditor.SerializedProperty property, UnityEditor.ObjectFieldValidator validator, Boolean allowSceneObjects, UnityEngine.GUIStyle style) (at C:/BuildAgent/work/d3d49558e4d408f4/Editor/Mono/GUI/ObjectField.cs:44) UnityEditor.EditorGUI.DoObjectField (Rect position, Rect dropRect, Int32 id, UnityEngine.Object obj, System.Type objType, UnityEditor.SerializedProperty property, UnityEditor.ObjectFieldValidator validator, Boolean allowSceneObjects) (at C:/BuildAgent/work/d3d49558e4d408f4/Editor/Mono/GUI/ObjectField.cs:12) UnityEditor.EditorGUI.ObjectField (Rect position, UnityEngine.Object obj, System.Type objType, Boolean allowSceneObjects) (at C:/BuildAgent/work/d3d49558e4d408f4/artifacts/EditorGenerated/EditorGUI.cs:2380) CourseBlockEdit.OnInspectorGUI () (at Assets/Editor/CourseBlockEdit.cs:20) UnityEditor.InspectorWindow.DrawEditors (Boolean isRepaintEvent, UnityEditor.Editor[] editors, Boolean eyeDropperDirty) (at C:/BuildAgent/work/d3d49558e4d408f4/Editor/Mono/Inspector/InspectorWindow.cs:850) UnityEditor.DockArea:OnGUI()

Edit: if it matters I am not using GUILayout.objectfeild() I am using GUI.objectFeild()

Hello I want to create a series of 75 x 75 squares in the editor environment by giving the length and width value. How can this be? Thanks