Can't find type mismatch

I am trying to run the following code but I keep getting the error "Run-time error 91: Object variable or With block variable not set"

    numberOfRows = Application.CountA(Range("A:A"))
    
    Application.CutCopyMode = False
    
    Dim numberOfRemainingDuplicates As Integer
    Dim lastOne As Variant
    Dim workingRange As Variant

    numberOfRemainingDuplicates = numberOfRows - 1
    lastOne = "A1"
    workingRange = "AF3:AF" & numberOfRows
    
    Do
        Range(workingRange).Select
        Selection.Find(What:="1", After:=ActiveCell, LookIn:=xlValues, LookAt:= _
        xlWhole, SearchOrder:=xlByRows, SearchDirection:=xlNext, MatchCase:=False _
        , SearchFormat:=False).Activate
        lastOne = ActiveCell.Address
        ActiveCell.EntireRow.Select
        Selection.Insert Shift:=xlDown
        
        numberOfRows = numberOfRows + 1
        Range(lastOne).Select
        Selection.Offset(2, 0).Select
        workingRange = ActiveCell.Address & ":$AF$" & numberOfRows
        numberOfRemainingDuplicates = Application.CountA(Range(workingRange))
        
    Loop While numberOfRemainingDuplicates > 1

(I highlighted in brown the portion of the code that gets highlighted when I enter debugging mode)

I have tried to introduce workingRange as

Dim workingRange As Range

and then removing the Range functions from workingRange in the rest of the code, but then I get other errors. What confuses me the most is that before I saved and closed the workbook that contains this macro, I ran the macro once successfully, so I find it strange that this error would only appear now?

I hope someone can see what is wrong with this code?