Using Unity's ScriptableWizards as Dialogs in CustomEditors.

AddThingToSomeScriptableObject.cs

public class AddThingToSomeScriptableObject : ScriptableWizard
{
    internal SomeScriptableObject someScriptableObject;
    public SomeThing someThing;

    void OnCreateButton()
    {
      //add someThing to someScriptableObject.
    }
}

SomeScriptableObjectEditor.cs

[CustomEditor(typeof(SomeScriptableObject))]
public class SomeScriptableObjectEditor : Editor
{
    public override void OnInspectorGUI()
    {
        var someScriptableObject = target as SomeScriptableObject;
        if (GUILayout.Button("Add a New Thing"))
        {
            var wizard = ScriptableWizard.DisplayWizard<AddThingToSomeScriptableObject>("New Thing", "Create");
            wizard.someScriptableObject = someScriptableObject;
        }
    }
}