Welcome to the world of MS/VB's Famous Red Herrings (aka error messages).
The problem is this: some shapes don't have a text frame (the property of a shape that holds text) and even if a shape has a text frame, there may not be any text in it. Attempting to change text that isn't there, or text in a text frame that isn't there will throw an error.
Use this instead, which tests for text frame and if present, whether the text frame has text prior to changing the text in any way:
Sub changenotestowhite()
Dim osld As Slide
Dim oshp As Shape
Dim strNotes As String
For Each osld In ActivePresentation.Slides
For Each oshp In osld.NotesPage.Shapes
If oshp.HasTextFrame Then
If oshp.TextFrame.HasText Then
oshp.TextFrame.TextRange.Font.Color = vbWhite
End If
End If
Next oshp
Next osld
End Sub