How to fix "object/block variable not set" error in VBA?

I'm using VBA in Word 2013.  My OS is Win10.

I'm having a problem with macros that I created that will no longer work.  When I click to run them, I get: "Run-time error '91': Object variable or With block variable not set."

I have no idea what the problem could be.  These macros ran fine before.  Then, this started happening.

For example, here's a very simply-written one to zoom out in order to be able to see 5 pages at once in Word:

Sub Five_Page_View()
'
' Zoom 45% to see 5 pages
'
'
    ActiveWindow.ActivePane.View.Zoom.Percentage = 45
End Sub

Used to work.  Now, doesn't.  I get the error above.  Why?

Does anyone have any idea what the problem might be?

Thanks!