Fixed a nullref when adding a new effect in a larger scene.

Adding a new effect can cause an infrequent nullreference: https://streamable.com/bk03b. Only seems to happen with large scenes, and only the first effect you add. The whole stacktrace is:

```
NullReferenceException: (null)
UnityEditor.SerializedObject..ctor (UnityEngine.Object obj) (at C:/buildslave/unity/build/Editor/Mono/SerializedObject.cs:10)
UnityEditor.Rendering.PostProcessing.PostProcessEffectBaseEditor.Init (UnityEngine.Rendering.PostProcessing.PostProcessEffectSettings target, UnityEditor.Editor inspector) (at Assets/Plugins/packages/PostProcessing/Editor/PostProcessEffectBaseEditor.cs:32)
UnityEditor.Rendering.PostProcessing.EffectListEditor.CreateEditor (UnityEngine.Rendering.PostProcessing.PostProcessEffectSettings settings, UnityEditor.SerializedProperty property, Int32 index) (at Assets/Plugins/packages/PostProcessing/Editor/EffectListEditor.cs:86)
UnityEditor.Rendering.PostProcessing.EffectListEditor.AddEffectOverride (System.Type type) (at Assets/Plugins/packages/PostProcessing/Editor/EffectListEditor.cs:231)
UnityEditor.Rendering.PostProcessing.EffectListEditor+<OnGUI>c__AnonStorey1.<>m__0 () (at Assets/Plugins/packages/PostProcessing/Editor/EffectListEditor.cs:189)
UnityEditor.GenericMenu.CatchMenu (System.Object userData, System.String[] options, Int32 selected) (at C:/buildslave/unity/build/artifacts/generated/common/editor/GenericMenuBindings.gen.cs:121)
```

The AddEffectOverride method calls AssetDatabase.SaveAssets, which causes the effect to be nullified. Probably there is some reloading going on?
https://streamable.com/nlujr

Moved the SaveAssets to the end of the method, added a comment to explain what and why.

Author: TJHeuvel

PostProcessing/Editor/EffectListEditor.cs

@@ -214,17 +214,17 @@ void AddEffectOverride(Type type)
             var effectProp = m_SettingsProperty.GetArrayElementAtIndex(m_SettingsProperty.arraySize - 1);
             effectProp.objectReferenceValue = effect;
 
-            // Force save / refresh
+            // Create & store the internal editor object for this effect
+            CreateEditor(effect, effectProp);
+
+            m_SerializedObject.ApplyModifiedProperties();
+
+            // Force save / refresh. Important to do this last because SaveAssets can cause effect to become null!
             if (EditorUtility.IsPersistent(Asset))
             {
                 EditorUtility.SetDirty(Asset);
                 AssetDatabase.SaveAssets();
             }
-
-            // Create & store the internal editor object for this effect
-            CreateEditor(effect, effectProp);
-
-            m_SerializedObject.ApplyModifiedProperties();
         }
 
         void RemoveEffectOverride(int id)