Can I open EditorWindow from CustomInspector ?

When I try to open EditorWindow from CustomInspector, I have such errors :

NullReferenceException: Object reference not set to an instance of an object UnityEngine.GUIUtility.GetControlID (Int32 hint, FocusType focus) (at E:/BuildAgent/work/68355d6e5d19d587/Runtime/Export/Generated/GUIUtility.cs:250) UnityEditor.InspectorWindow.OnGUI () (at E:/BuildAgent/work/68355d6e5d19d587/Editor/Mono/Inspector/InspectorWindow.cs:231) System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture)

Something wrong with GUILayout. The window opens, but I allways see this error. How can do it without abuse in the console?

Not sure what is happening here, but one thing you can try is to call the undocumented function GUIUtility.ExitGUI(), right after opening the window. Possibly, the opening of the window upsets the state of the GUI layout system, so you'd just exit the GUI from there, and have it all fixed in the next layout event.