Thank you for the provided image.
To avoid any misunderstanding, may I ask you to elaborate more on the steps to reproduce this behavior. You open the dropdown of the RadMultiColumnComboBox, and exceptions occur, or there is an additional step? My guess here is that you are using implicit style. This style will be applied to the RadGridView inside the dropdown of the GridViewMultiColumnComboBoxColumn when it is open. Can you try instead of setting this property in style to place it directly to the parent RadGridView and check again if this exception appears?
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