The current Spreadsheet control does not provide support for editing the RichText cells via the FormulaBar. Hence you can notify the end users through the MessageBox.
When you try to edit the RichText cells via formula bar, a message box is displayed saying it cannot be edited through the FormulaBar.
This can be achieved by handling the PreviewMouseLeftButtonDown event of the FormulaBar. The FormulaBar of Spreadsheet control can be obtained by using the static method GetFieldvalue, Object, of ReflectionHelper class.
For your reference, a sample is created with ReflectionHelper class to show the MessageBox while trying to edit FormulaBar.
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