AssetImportContext seems not saved added assets

https://docs.unity3d.com/ScriptReference/Experimental.AssetImporters.ScriptedImporter.html

I add a custom asset importer, according the sample code in the above manual.
I got the error blow
4223320--375301--upload_2019-2-17_2-12-15.png

sometimes, i can get the right content.
And the asset,
ctx.AddObjectToAsset(“main obj”, asset);
ctx.SetMainObject(asset);
after enter once Editor PlayMode, the mainObject i loaded, is not my added asset anymore.
I need reimport it again.

Unity 2018.3.6f1

CustomAsset : ScriptableObject
need locate in the same name file: CustomAsset.cs