When writing Unity tools, if you rely too much on GUILayout or EditorGUILayout,
Excessive use of functions such as BeginHorizontal and BeginVertical
It is inevitable that logically sometimes they and the corresponding end function do not appear in pairs.
Errors similar to the following are because there is an end function but no start function
That is, EndVertical() is written but there is no BeginVertical()
You need to check carefully in the code,
NullReferenceException: Object reference not set to an instance of an object
UnityEngine.GUILayoutUtility.LayoutFromContainer (Single w, Single h) (at C:/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayoutUtility.cs:182)
UnityEngine.Experimental.UIElements.UIElementsUtility.EndContainerGUI () (at C:/buildslave/unity/build/Runtime/UIElements/Managed/UIElementsUtility.cs:148)
UnityEngine.Experimental.UIElements.IMGUIContainer.DoOnGUI (UnityEngine.Event evt) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/IMGUIContainer.cs:140)
UnityEngine.Experimental.UIElements.IMGUIContainer.HandleEvent (UnityEngine.Event evt, UnityEngine.Experimental.UIElements.VisualElement finalTarget) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/IMGUIContainer.cs:185)
UnityEngine.Experimental.UIElements.IMGUIContainer.DoRepaint (IStylePainter painter) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/IMGUIContainer.cs:38)
UnityEngine.Experimental.UIElements.Panel.PaintSubTree (UnityEngine.Event e, UnityEngine.Experimental.UIElements.VisualElement root, Matrix4x4 offset, Rect currentGlobalClip) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/Panel.cs:425)
UnityEngine.Experimental.UIElements.Panel.PaintSubTree (UnityEngine.Event e, UnityEngine.Experimental.UIElements.VisualElement root, Matrix4x4 offset, Rect currentGlobalClip) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/Panel.cs:435)
UnityEngine.Experimental.UIElements.Panel.Repaint (UnityEngine.Event e) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/Panel.cs:450)
UnityEngine.Experimental.UIElements.UIElementsUtility.DoDispatch (UnityEngine.Experimental.UIElements.BaseVisualElementPanel panel) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/UIElementsUtility.cs:178)
UnityEngine.Experimental.UIElements.UIElementsUtility.ProcessEvent (Int32 instanceID, IntPtr nativeEventPtr) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/UIElementsUtility.cs:71)
UnityEngine.GUIUtility.ProcessEvent (Int32 instanceID, IntPtr nativeEventPtr) (at C:/buildslave/unity/build/Runtime/IMGUI/Managed/GUIUtility.cs:186)