Hello!

The given samples call functions that are only meant to be called in "OnGui" in a function that is not "OnGui" (GetHeight); this is not guaranteed to work.
This limitation can be worked-around by not using those functions in GetHeight. To achieve this, you can either use alternatives that can be called outside of OnGui or cache some results. Here is an example updated HelpBoxAttributeDrawer:

[CustomPropertyDrawer(typeof(HelpBoxAttribute))]
public class HelpBoxAttributeDrawer : DecoratorDrawer
{
float m_lastWidth;

public override float GetHeight()
{
if (attribute is not HelpBoxAttribute helpBoxAttribute)
return base.GetHeight();
return EditorStyles.helpBox.CalcHeight(new GUIContent(helpBoxAttribute.text), m_lastWidth);
}

public override void OnGUI(Rect position)
{
var helpBoxAttribute = attribute as HelpBoxAttribute;
if (helpBoxAttribute == null) return;
m_lastWidth = EditorGUIUtility.currentViewWidth;
EditorGUI.HelpBox(position, helpBoxAttribute.text, GetMessageType(helpBoxAttribute.messageType));
}
...
}

There is a separate issue that causes the inspector to not take the height of this decorator into account if it varies, such as if you resize the inspector. This is a known issue that will be fixed.

In general, please note that starting 2022.1, the recommended way to create editor UI is with UIToolkit and that it might make sense for you to migrate from IMGUI.