I am getting this error on Unity 2021.3.8 LTS with UnityEditorInternal.ReorderableList. Would be nice to get a fix there also.

I having the same issuses with the TerrainTool from Unity-Build-In packages, I using Unity Version 2021.3.1f1and my solution is:
Add the following two lines in the method from the FilterStackView class found in TerrainTool's FilterStackView.cs:
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private Filter GetFilterAtIndex(int index)
{
// Add these next two lines in this Methods
if (index >= m_FiltersProperty.arraySize)
return null;

return m_FiltersProperty.GetArrayElementAtIndex(index).objectReferenceValue as Filter;
}
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