Font members of `EditorStyles` is no longer using backing fields since 2019.3

[EmiyaSyahriel]

Since Unity 2019.3, the properties to get fonts (e.g EditorStyles.standardFont) is changed from getting the value from a backing field to calling EditorResources.GetFont(FontDef.Style.[style]), and the backing field seems to be removed. Changes : UnityCsReference 2019.2 -> UnityCsReference 2019.3

Therefore these lines :

ModifyEditorStyle/Editor/ModifyEditorStyle.cs

Lines 318 to 325 in 1fa5ecb

	//EditorStyles static was pulled from s_Current which was populated from `EditorGUIUtility.GetBuiltinSkin` which we cannot interfere.
	//s_Current is internal and therefore we need to reflect to change the font. All other styles are accessible except the fonts.
	var eType = typeof(EditorStyles);
	var es = (EditorStyles)(eType.GetField("s_Current", BindingFlags.Static | BindingFlags.NonPublic).GetValue(null));
	eType.GetField("m_StandardFont", BindingFlags.Instance | BindingFlags.NonPublic).SetValue(es, normalFont);
	eType.GetField("m_BoldFont", BindingFlags.Instance | BindingFlags.NonPublic).SetValue(es, boldFont);
	eType.GetField("m_MiniFont", BindingFlags.Instance | BindingFlags.NonPublic).SetValue(es, smallFont);
	eType.GetField("m_MiniBoldFont", BindingFlags.Instance | BindingFlags.NonPublic).SetValue(es, smallBoldFont);

will throw either NullPointerException or FieldNotFoundException and the code will no longer works.