How to use two workbooks and their worksheets altogether - vba

Question

I am using two workbooks at a time. The first workbook is the current workbook and the second will be opened while programming execution. I have made the global objects of the workbooks and worksheets. I'm having issues with using the worksheets simultaneously. The error is ERROR: object variable or with block variable, not set. I have mentioned the error in the comment in the second subroutine.
Dim WB As Workbook
Dim WB2 As Workbook
Dim WKS As Worksheet
Dim WKS2 As Worksheet
Sub Button1_Click()
Dim fd As Office.FileDialog
Set fd = Application.FileDialog(msoFileDialogFilePicker)
Set WB = ThisWorkbook
WB.Activate
fd.AllowMultiSelect = False
fd.Title = "Provide a title here."
fd.InitialFileName = ThisWorkbook.Path
If fd.Show() = True Then
strFile = fd.SelectedItems(1)
Set WB2 = Workbooks.Open(strFile)
Set WSK2 = WB2.Sheets("Schweitzer Analysis")
CalculateGrades
Else
MsgBox ("No file selected")
End If
End Sub
Sub CalculateGrades()
' first clear the contents where grades results can appear
clearGradesContents
Dim index As Integer ' for current sheet
Dim index2 As Integer ' for student sheet
Dim countCorrect As Integer ' to count no of correct answers
index = 2
index = 8
countCorrect = 0
' this first error here
' ERROR: object variable or with block variable not set
Set WKS = WB.ActiveSheet
Do While index <= 21
' the SECOND error shows here
If WKS.Cells(index, 2) = WKS2.Cells(index2, 3) Then
Count = Count + 1
Else
WKS.Cells(index, 5) = WKS2.Cells(index2, 3)
End If
If WKS.Cells(index, 3) = WKS2.Cells(index2, 4) Then
Count = Count + 1
Else
WKS.Cells(index, 6) = WKS2.Cells(index2, 4)
End If
index2 = index2 + 1
index = index + 1
Loop
End Sub
Sub clearGradesContents()
Range("E2:F21").Select
Selection.ClearContents
Range("I2:I3").Select
Selection.ClearContents
End Sub

Answer 1

EDIT: sorry for the previous answer, I hadn't seen your global declarations on top and I was sure it was because you hadn't declare it.
I think the issue is the following:
[...]
Set WB = ThisWorkbook
WB.Activate '<-- WB is the active workbook now
[...]
Set WB2 = Workbooks.Open(strFile) '<-- WB2 is the active workbook now!
Set WSK2 = WB2.Sheets("Schweitzer Analysis")
CalculateGrades '<-- going to next macro
[...]
Set WKS = WB.ActiveSheet '<-- ERROR: WB2 is the active workbook. WB can't have an active sheet if it's not the active workbook
Then of course, being WKS not correctly set, you can't execute this:
If WKS.Cells(index, 2) = WKS2.Cells(index2, 3) Then
I have no way to test it now, but I think that should be the issue.
To solve it,
If you really need WKS to be the ActiveSheet
Dim activeSheetName As String
...
Sub Button1Click()
...
Set WB = ThisWorkbook
WB.Activate
activeSheetName = WB.ActiveSheet.Name
...
End Sub
Sub CalculateGrades()
...
Set WKS = WB.Sheets(activeSheetName)
...
If you already know the name of the sheet you need
Just write:
Set WKS = WB.Sheets("your sheet")
instead of
Set WKS = WB.ActiveSheet