NLog version: 6.0

Platform: .NET8

Current NLog config (xml or C#, if relevant)

<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      autoReload="true"

      throwConfigExceptions="false"
      internalLogLevel="Trace"
      internalLogFile="${basedir}/logs/internal-nlog.txt">

	<targets>
		<target xsi:type="RichTextBox"
				name="EventToRichTextFieldGUI"
				layout="${longdate} | ${uppercase:${level}} | ${message:withexception=false}"
				controlName="LogTextBox"
				formName="Form1"
				autoScroll="true"
				maxLines="1000"
				useDefaultRowColoringRules="True"/>
	</targets>

	<rules>
		<logger name="*" minlevel="Trace" writeTo="EventToRichTextFieldGUI"/>
	</rules>

</nlog>

• What is the current result?
  NLog fails to write to RichTextBox with DefaultRowColoringRules

• What is the expected result?
  Default colored text in my RichTextBox

• Did you checked the Internal log?
  yes.

• Please post full exception details (message, stacktrace, inner exceptions)

Warn RichTextBoxTarget(Name=EventToRichTextFieldGUI): Exception in Write Exception: System.TypeInitializationException: The type initializer for 'NLog.Windows.Forms.RichTextBoxTarget' threw an exception.
---> System.TypeInitializationException: The type initializer for 'NLog.Windows.Forms.RichTextBoxRowColoringRule' threw an exception.
---> System.ArgumentNullException: Value cannot be null. (Parameter 'expressionText')
at NLog.Internal.Guard.ThrowIfNull[T](T arg, String param)
at NLog.Conditions.ConditionParser.ParseExpression(String expressionText)
at NLog.Conditions.ConditionExpression.op_Implicit(String conditionExpressionText)
at NLog.Windows.Forms.RichTextBoxRowColoringRule..ctor(String condition, String fontColor, String backColor, FontStyle fontStyle)
at NLog.Windows.Forms.RichTextBoxRowColoringRule..ctor()
at NLog.Windows.Forms.RichTextBoxRowColoringRule..cctor()
--- End of inner exception stack trace ---
at NLog.Windows.Forms.RichTextBoxRowColoringRule..ctor(String condition, String fontColor, String backColor, FontStyle fontStyle)
at NLog.Windows.Forms.RichTextBoxTarget.CreateDefaultColoringRules()
at NLog.Windows.Forms.RichTextBoxTarget..cctor()
--- End of inner exception stack trace ---
at NLog.Windows.Forms.RichTextBoxTarget.get_DefaultRowColoringRules()
at NLog.Windows.Forms.RichTextBoxTarget.FindMatchingRule(LogEventInfo logEvent)
at NLog.Windows.Forms.RichTextBoxTarget.Write(LogEventInfo logEvent)
at NLog.Targets.Target.Write(AsyncLogEventInfo logEvent)

• Are there any workarounds? yes/no
  I dont know. Maybe providing custom coloring rules.

• Is there a version in which it did work?
  yes, NLog <=5.5.0

• Can you help us by writing an unit test?