Object variable or with block variable not set

Hello, i made a code to read data from specific sheets of multiple workbooks. The error i get is the "object variable or with block variable not set". I made a some changes in the code but it did not worked. So here is the code:

Option Explicit

Dim wb1 As Workbook
Dim wb2 As Workbook
Dim wb3 As Workbook
Dim wb4 As Workbook

Sub ActivateAustria()
    Workbooks.Open Filename:="C:\Users\red\Desktop\1st\excel1.xls"
    Windows("excel1.xls").Activate
    wb1 = "excel1.xls"
    Workbooks.Open Filename:="C:\Users\ref\Desktop\1st\excel2.xls"
    Windows("excel2.xls").Activate
    wb2 = "excel2.xls"
    Workbooks.Open Filename:="C:\Users\ref\Desktop\1st\excel3.xls"
    Windows("excel3.xls").Activate
    wb3 = "excel3.xls"
    Workbooks.Open Filename:="C:\Users\ref\Desktop\1st\excel4.xls"
    Windows("excel4.xls").Activate
    wb4 = "excel4.xls"
    ActiveWindow.Close
    ActiveWindow.Close
    ActiveWindow.Close
    ActiveWindow.Close
    MsgBox ("ok")
End Sub

Sub ActivateHDB()

    Workbooks(w1).Worksheets("En_mob").Range("G:G").Activate

End Sub

I get the error in the 9th line where i state "wb1 =". I changed the variables into public instead of dim and nothing happened. I also changed the subs and made them public subs, nothing happened. I also stated the workbook parameters as <Set wb1 = New Workbook> also nothing happened :|
Any suggestions?