Default Resources are not serialised anymore in the LW pipeline asset. They come from resource asset and are also not exposed anymore.

ScriptableRenderPipeline/LightweightPipeline/Data/LightweightPipelineAsset.cs

         [SerializeField] private Shader m_CopyDepthShader;
 
 #if UNITY_EDITOR
-        [SerializeField] private Material m_DefaultMaterial;
-        [SerializeField] private Material m_DefaultParticleMaterial;
-        [SerializeField] private Material m_DefaultTerrainMaterial;
+        private LightweightPipelineResource m_ResourceAsset;
 
         [MenuItem("Assets/Create/Render Pipeline/Lightweight/Pipeline Asset", priority = CoreUtils.assetCreateMenuPriority1)]
         static void CreateLightweightPipeline()
                     resourceAsset = AssetDatabase.LoadAssetAtPath<LightweightPipelineResource>(path);
                 }
 
-                if (resourceAsset != null)
-                {
-                    instance.m_DefaultMaterial = resourceAsset.DefaultMaterial;
-                    instance.m_DefaultParticleMaterial = resourceAsset.DefaultParticleMaterial;
-                    instance.m_DefaultTerrainMaterial = resourceAsset.DefaultTerrainMaterial;
-                }
-
+                instance.m_ResourceAsset = resourceAsset;
                 instance.m_DefaultShader = Shader.Find(LightweightShaderUtils.GetShaderPath(ShaderPathID.STANDARD_PBS));
                 instance.m_BlitShader = Shader.Find(LightweightShaderUtils.GetShaderPath(ShaderPathID.HIDDEN_BLIT));
                 instance.m_CopyDepthShader = Shader.Find(LightweightShaderUtils.GetShaderPath(ShaderPathID.HIDDEN_DEPTH_COPY));
         public override Material GetDefaultMaterial()
         {
 #if UNITY_EDITOR
-            return m_DefaultMaterial;
-#else
+            if (m_ResourceAsset != null)
+                return m_ResourceAsset.DefaultMaterial;
+#endif
-#endif
-            return m_DefaultParticleMaterial;
-#else
-            return null;
+            if (m_ResourceAsset != null)
+                return m_ResourceAsset.DefaultParticleMaterial;
+            return null;
         }
 
         public override Material GetDefaultLineMaterial()
         public override Material GetDefaultTerrainMaterial()
         {
 #if UNITY_EDITOR
-            return m_DefaultTerrainMaterial;
-#else
-            return null;
+            if (m_ResourceAsset != null)
+                return m_ResourceAsset.DefaultTerrainMaterial;
+            return null;
         }
 
         public override Material GetDefaultUIMaterial()

ScriptableRenderPipeline/LightweightPipeline/Editor/LightweightAssetEditor.cs

         {
             public static GUIContent renderingLabel = new GUIContent("Rendering");
             public static GUIContent shadowLabel = new GUIContent("Shadows");
-            public static GUIContent defaults = new GUIContent("Default Materials");
 
             public static GUIContent renderScaleLabel = new GUIContent("Render Scale", "Scales the camera render target allowing the game to render at a resolution different than native resolution. UI is always rendered at native resolution. When in VR mode, VR scaling configuration is used instead.");
 
             public static GUIContent shadowCascadeSplit = new GUIContent("Cascades Split",
                     "Percentages to split shadow volume");
 
-            public static GUIContent defaultMaterial = new GUIContent("Mesh",
-                    "Material to use when creating 3D objects");
-
-            public static GUIContent defaultParticleMaterial = new GUIContent("Particles",
-                    "Material to use when creating Particle Systems");
-
-            public static GUIContent defaultTerrainMaterial = new GUIContent("Terrain",
-                    "Material to use in Terrains");
-
             public static GUIContent msaaContent = new GUIContent("Anti Aliasing (MSAA)", "Controls the global anti aliasing settings.");
 
             public static string[] shadowTypeOptions = {"No Shadows", "Hard Shadows", "Hard and Soft Shadows"};
         private SerializedProperty m_ShadowCascadesProp;
         private SerializedProperty m_ShadowCascade2SplitProp;
         private SerializedProperty m_ShadowCascade4SplitProp;
-        private SerializedProperty m_DefaultMaterial;
-        private SerializedProperty m_DefaultParticleMaterial;
-        private SerializedProperty m_DefaultTerrainMaterial;
         private SerializedProperty m_MSAA;
 
         void OnEnable()
             m_ShadowCascadesProp = serializedObject.FindProperty("m_ShadowCascades");
             m_ShadowCascade2SplitProp = serializedObject.FindProperty("m_Cascade2Split");
             m_ShadowCascade4SplitProp = serializedObject.FindProperty("m_Cascade4Split");
-            m_DefaultMaterial = serializedObject.FindProperty("m_DefaultMaterial");
-            m_DefaultParticleMaterial = serializedObject.FindProperty("m_DefaultParticleMaterial");
-            m_DefaultTerrainMaterial = serializedObject.FindProperty("m_DefaultTerrainMaterial");
             m_MSAA = serializedObject.FindProperty("m_MSAA");
         }
 
             EditorGUI.BeginChangeCheck();
+
+            if (mode >= options.Length)
+                Debug.LogError(string.Format("Invalid option while trying to set {0}", label.text));
+
             mode = EditorGUILayout.Popup(label, mode, options);
             if (EditorGUI.EndChangeCheck())
             {
                 EditorGUILayout.PropertyField(m_ShadowCascade2SplitProp, Styles.shadowCascadeSplit);
             }
 
-            EditorGUI.indentLevel--;
-
-            EditorGUILayout.Space();
-            EditorGUILayout.LabelField(Styles.defaults, EditorStyles.boldLabel);
-            EditorGUI.indentLevel++;
-            EditorGUILayout.PropertyField(m_DefaultMaterial, Styles.defaultMaterial);
-            EditorGUILayout.PropertyField(m_DefaultParticleMaterial, Styles.defaultParticleMaterial);
-            EditorGUILayout.PropertyField(m_DefaultTerrainMaterial, Styles.defaultTerrainMaterial);
             EditorGUI.indentLevel--;
 
             serializedObject.ApplyModifiedProperties();

Tests/GraphicsTests/RenderPipeline/LightweightPipeline/LightweightPipelineAsset.asset

   m_ShadowAtlasResolution: 2048
   m_ShadowNearPlaneOffset: 2
   m_ShadowDistance: 50
-  m_ShadowCascades: 4
+  m_ShadowCascades: 0
-  m_DefaultMaterial: {fileID: 2100000, guid: 31321ba15b8f8eb4c954353edc038b1d, type: 2}
-  m_DefaultParticleMaterial: {fileID: 2100000, guid: e823cd5b5d27c0f4b8256e7c12ee3e6d,
-    type: 2}
-  m_DefaultTerrainMaterial: {fileID: 2100000, guid: 594ea882c5a793440b60ff72d896021e,
-    type: 2}