Previews work properly again and switch between 2d and 3d properly.

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/DrawData/NodePreviewDrawData.cs

                 bool status = false;
                 if (m_modificationScope == ModificationScope.Graph)
                 {
+                    previewGenerator.Reset();
+
                     // TODO: Handle shader regeneration error
                     status = UpdatePreviewShader();
                 }

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/MaterialGraphPreviewGenerator.cs

         private static readonly GUIContent[] s_LightIcons = {null, null};
         private static readonly GUIContent[] s_TimeIcons = {null, null};
 
-        public MaterialGraphPreviewGenerator()
+        private PreviewRenderUtility utility
-            if (m_PreviewUtility == null)
+            get
-                m_PreviewUtility = new PreviewRenderUtility();
-                EditorUtility.SetCameraAnimateMaterials(m_PreviewUtility.m_Camera, true);
+                if (m_PreviewUtility == null)
+                    m_PreviewUtility = new PreviewRenderUtility();
+
+                return m_PreviewUtility;
+        }
+        public void Reset()
+        {
+            if (m_PreviewUtility != null)
+                m_PreviewUtility.Cleanup();
+
+            m_PreviewUtility = null;
+        }
+
+        public MaterialGraphPreviewGenerator()
+        {
+            EditorUtility.SetCameraAnimateMaterials(utility.m_Camera, true);
+         
             if (s_Meshes[0] == null)
             {
                 var handleGo = (GameObject)EditorGUIUtility.LoadRequired("Previews/PreviewMaterials.fbx");
             if (mat == null || mat.shader == null)
                 return Texture2D.blackTexture;
 
-            m_PreviewUtility.BeginPreview(size, GUIStyle.none);
+            utility.BeginPreview(size, GUIStyle.none);
-                m_PreviewUtility.m_Camera.transform.position = -Vector3.forward * 5;
-                m_PreviewUtility.m_Camera.transform.rotation = Quaternion.identity;
+                utility.m_Camera.transform.position = -Vector3.forward * 5;
+                utility.m_Camera.transform.rotation = Quaternion.identity;
-                m_PreviewUtility.m_Camera.projectionMatrix = Matrix4x4.identity;
+                utility.m_Camera.projectionMatrix = Matrix4x4.identity;
-			EditorUtility.SetCameraAnimateMaterialsTime(m_PreviewUtility.m_Camera, time);
-			m_PreviewUtility.m_Light[0].intensity = 1.0f;
-			m_PreviewUtility.m_Light[0].transform.rotation = Quaternion.Euler(50f, 50f, 0);
-			m_PreviewUtility.m_Light[1].intensity = 1.0f;
-			InternalEditorUtility.SetCustomLighting(m_PreviewUtility.m_Light, Color.black);
+			EditorUtility.SetCameraAnimateMaterialsTime(utility.m_Camera, time);
+            utility.m_Light[0].intensity = 1.0f;
+            utility.m_Light[0].transform.rotation = Quaternion.Euler(50f, 50f, 0);
+            utility.m_Light[1].intensity = 1.0f;
+			InternalEditorUtility.SetCustomLighting(utility.m_Light, Color.black);
-			m_PreviewUtility.m_Camera.clearFlags = CameraClearFlags.SolidColor;
+            utility.m_Camera.clearFlags = CameraClearFlags.Depth;
-			m_PreviewUtility.DrawMesh(
+            utility.DrawMesh(
-			m_PreviewUtility.m_Camera.Render();
+            utility.m_Camera.Render();
-            return m_PreviewUtility.EndPreview();
+            return utility.EndPreview();
         }
     }
 }