Calling Function from Module to Form got an error Variable not set

just want to know what I missed with these. I'm calling sub from mainwindow

in MainWindow Form

Code:

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Public Sub ChangeData()
      msgbox("Ok")
End Sub

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in Module

Code:

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Sub PrintIntercept()
      Mainwindow.ChangeData  '------> Here it says that Object variable or With block variable not set
End Sub

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bump