Subscript Out Of Range when accessing Sheet in another opened Workbook - vba

Question

I want to reference to the first opened Workbook. 2 workbooks are open: one that is calling the macro to execute, and the workbook which contains the macro. Somehow, the code runs smoothly very often. But sometimes, there occurs a referencing error when accessing data in the first opened workbook: Subscript Out Of Range.
On this line, the error occurs:
Set mastersheet = Workbooks(1).Sheets("Setting")
So there should be two workbooks in the collection. What am I overseeing? Please keep in mind, that the first opened workbook doesn't have a fixed name, so the name of the open workbook changes. The second Workbook, the workbook which contains the macro to execute, doesn't change it's name.

Answer 1

Unfortunately, you don't provide more code or any information about what kind of code "container" the macro is in. The following solution assumes the macro is in a Worksheet or Workbook code container (Sheet1 or ThisWorkbook in the VBA Editor, for example).
It's possible to get the workbook from the code container. If the code is in a Worksheet code container, then use Me.Parent. If it's in ThisWorkbook use Me. These containers are actually classes that represent the Worksheet / the Workbook object. So Me refers to that object. The parent of a Worksheet is its workbook.
So a Workbook object is set to its container workbook. Then the open workbooks are looped in a For...Each and the workbook is tested whether it's the same as the code container workbook, or if it's another. If it's another, then the loop is exited. Debug.Print shows the result (two different names) and demonstrates how to continue to work with the separate workbook objects.
Sub GetOtherWorkbook()
Dim wbWithMacro As Workbook
Dim wbOther As Workbook, wb As Workbook
Set wbWithMacro = Me.Parent 'Assumes macro is in a "Sheet" code container
'Set wbWithMacro = Me 'Assumes macro is in "ThisWorkbook" code container
For Each wb In Workbooks
If Not wb Is Me Then
Set wbOther = wb
Exit For
End If
Next
Debug.Print wbWithMacro.Name, wbOther.Name
End Sub

Answer 2

I suggest the following to find the Setting worksheet:
Option Explicit
Public Sub FindSettingWorksheet()
Dim MasterSheet As Worksheet
Dim wb As Workbook
For Each wb In Workbooks 'loop through all open workbooks
On Error Resume Next 'stop error reporting
Set MasterSheet = wb.Worksheets("Setting") 'if this throws an error it's the wrong workbook
On Error GoTo 0 're-enable error reporting
If Not MasterSheet Is Nothing Then Exit For 'if we found the setting worksheet we can exit/stop
Next wb
If Not MasterSheet Is Nothing Then 'test if we found it
Debug.Print MasterSheet.Name
Else
Debug.Print "Settings not found"
End If
End Sub