Subscript out of Range

Hi Experts,

My requirement is explained below
step 1: (Myfile) is active workbook-->Click Button-->open the Required workbook file -->from that file ,save all sheet name in combo-list box
step 2: Now select sheet name from combo box-->selected sheetname will be paste to "myfile" workbook




Mycode is below

sub maincodeButton_click()
Call module1.callsheet
Call module1.tocopysheet
end sub

-------------------------------------------------------------------




'(Module1 code)

Option Explicit
Public filename As Variant, ddlfilename As Variant
-----------------------------------------------------

'To store sheets name in combo box selection
Public Sub callsheet()

Dim myname(1 To 40) As String
Dim icount As Integer
Dim sheetcount As Integer

Sheet2.sheetnameComboBox.Clear

'opening the workbook file
filename = Application.GetOpenFilename(, , "Select Programme")
Workbooks.Open filename
filename = Mid(filename, InStrRev(filename, "\") + 1)
Application.Workbooks(filename).Activate

'store sheet name in the array variable and put into combo box
sheetcount = Sheets.Count
    For icount = 1 To sheetcount
        myname(icount) = Sheets(icount).Name
        With Sheet2.sheetnameComboBox
        .AddItem myname(icount)
        End With
Next
End Sub

---------------------------------------------------------

'copy that particular tab in another file

Sub tocopysheet()
    Windows(filename).Activate
    Sheets(Sheet2.sheetnameComboBox.Value).Select
    Cells.Select
    Selection.Copy
    Windows("myfile").Activate
    Sheets("sheet1").Select
    Range("A1").Select
    ActiveSheet.Paste
    Windows(filename).Close
    
End Sub

------------------------------------------------------------

My problem is..

for First time run --> code ran fine
for Second time run--> am getting error "Subscript out of Range"
only if i click reset button I am able to run


This error because Windows(filename).Activate ,here where variable "filename" was initially empty while calling sub maincodeButton_click()
Always for second run I have to Reset macro .....But i dont know how to over come...please solve my problem